Meanwhile, as investigators on three continents worked to flesh out details of the plot, Pakistani officials reported they had arrested as many as 17 more suspected conspirators in recent days. One of those, a British national named Rashid Rauf, is believed to have been the operational planner and to have connections with Al Qaeda in Afghanistan, Pakistani and U.S. officials told Associated Press.

At least three of those arrested in Britain are converts to Islam, one just within the last six or seven months, and another one is a security guard at Heathrow airport, the scene of passenger chaos on Thursday as hundreds of flights were cancelled as authorities came to grips with drastic new security measures aimed at keeping planes safe in the air.

No details have been released about the two women arrested, one of whom is reported to be pregnant while the other is the mother of an infant. Both are believed to be the wives of two of the accused men. Those arrested range in age from 17 to 35.

One of the arrested men, Waheed Zaman, 23, is a well-known Muslim representative on the student council at London's Metropolitan University, where he studies biochemistry.

His sister, who was with him in the family's east London house when police arrived to arrest her brother, told Britain's Sun newspaper he is a mainstream Muslim who is a proud Briton.

"He loves fish and chips and Liverpool football club," she said, adding that he is a "great believer in the importance of integration between our community and the Western world."

But for the better part of a year, police here have been shadowing the suspects as they went about their seemingly normal "fish and chip" lives, tracking their cellphone conversations, email and Internet access, bank accounts and even using GPS tracking devices on at least 12 cars belonging to suspects.

Details of how police managed to break up an alleged suicidal plan to detonate disguised liquid explosives aboard as many as 10 planes bound from Britain to the U.S. continued to emerge yesterday - although British authorities remain reluctant to disclose too much information, sensitive to the justice system still to be navigated once the terror suspects are formally charged.

Media reports in Britain suggested the investigation into the bomb plot goes back almost a year after a tip-off from an informant in east London's Muslim community that led police to begin monitoring Internet traffic among several men. At least three of those men were already on the police radar, suspected of extremist tendencies.

What has yet to emerge is how the various suspects might be connected to one another. While more than half of those arrested come from the same east London suburb, there is no obvious connection between them and the group of four arrested in the north London suburb of High Wycombe or the two arrested in Birmingham.

Yesterday, police seized hard drives from Internet cafes in Reading and east London as they continued to gather vast amounts of evidence. The record of financial transactions, along with phone and computer records, could help investigators trace more people involved.

In recent months, police here have confirmed they believe there are as many as 1,000 Muslims living in Britain who sympathize with suicide bombings against western targets and who could be persuaded to join jihadist groups. In December, the probe intensified as Britain's domestic spy agency MI5 called in the anti-terrorist branch of Scotland Yard to help.

The Guardian newspaper reported police decided to end their surveillance operation and launch the raids Wednesday night after a "go" message was forwarded to one of the alleged terrorists by an accomplice in Pakistan. The coded message, which the Guardian said was deciphered by the spy agency, followed the transfer of large amounts of money to the suspects - money investigators believe was to be used to buy airline tickets for would-be bombers.

Other media reports suggest police intercepted plans for a "dry run" to be carried out yesterday whereby the bombers would have carried the ingredients for liquid bombs through airport security to test whether their false-bottomed energy drink and pop bottles raised suspicion. The Times newspaper said police felt they could not risk letting the bombers take another step closer to endangering planes and launched their simultaneous raids.

London's Evening Standard reported the plotters apparently chose next Wednesday as a target date, since they had tickets for a United Airlines flight that day. There were signs preparations stepped up recently. One of the houses raided by British police this week had been bought last month by two men in an all-cash deal, in a neighbourhood of $300,000 houses, neighbours said.

Early yesterday, the Bank of England website posted the names of 19 of the men in custody and stated their assets had been frozen by order of the government. The move came as the police followed the routine practice of not formally naming the suspects until they are officially charged. One of the 24 detainees was freed later yesterday, but Scotland Yard didn't identify that person.

Pakistani officials, meanwhile, said British information led to the first arrests in Pakistan about a week ago, of two British nationals, including Rauf, called a "key person" by the Pakistani foreign ministry.

Elsewhere, police in Italy raided Internet cafes, money-transfer offices and long-distance phone call centres catering to Muslims and arrested 40 people in a crackdown linked to Britain's announcement it had foiled the plot, authorities said.

On the streets of the east London neighbourhood of Walthamstow yesterday, young Muslim men gathered to denounce the arrests and voice skepticism that their neighbours might be capable of what police describe as "mass murder on an unimaginable scale."

"They've got the wrong guys. I know they have," said Amir Gull, who said he works nearby. "I know these people and I know that they are peaceful people. They would not do this."

Others rallied around to point out that just two months ago, police raided a nearby house and arrested two brothers as terrorist suspects, accidentally shooting one of them in the shoulder. A week later, both were released without charge and police confirmed they acted on false intelligence.

Almost all of the latest British suspects still lived with their immigrant parents in working-class communities and held various part- or full-time jobs while attending prayers at mosque regularly. Only one was attending university and almost all wore traditional Islamic dress.

Amin Asmin Tariq, 23, was the exception. The Heathrow Airport security guard dressed in western-style clothing and was not considered by friends or neighbours to be religious. He is married with a young child. Airport authorities said Tariq was not assigned to passenger screening.

Muslim convert Oliver Savant, 26, was recently married to a Muslim woman and runs a business with his brother, who told the Sun newspaper yesterday that Savant, who now goes by the first name Ibrahim, is a loyal Briton. It was believed Savant's pregnant wife is among the suspects being held by police.

In the upscale London suburb of High Wycombe, police evacuated homes and businesses around one raided business, where a string of garages in the back were cordoned off, leading to suspicion the property was being used as the bomb factory.

Police also arrested recent convert Don Stewart-Whyte, 21, as he arrived home with is wife. Stewart-Whyte, who changed his name to Abdul Waheed after his conversion early this year, is the half-brother of a former supermodel. His mother is a local high school teacher and his late father was a Conservative party worker.

At Britain's airports yesterday, flights were getting back to normal after a long day of confusion on Thursday that saw hundreds of flights cancelled. A spokesperson for BAA, which operates several major airports including London's Heathrow, Gatwick and Stansted, said about 70 per cent of domestic and short-haul flights would be taking off, but warned of continued delays.

The names released are of British citizens with Muslim names, many common in Pakistan. But as police patrolled in Pakistani neighborhoods outside of London, many here just didn't believe the story. But some are willing to wait for the evidence.

The men arrested yesterday were British, but sources say they had handlers from both Karachi and Lahour in Pakistan. As many as 10 people have been arrested there as facilitators of the plot, and when they were arrested late last week, others allegedly sent that message to great Britain: do your attacks now.

One of those arrested in Pakistan, Rashid Rauf, is believed to be the mastermind of this plot, a man with strong ties to Al Qaeda and who was found as he was headed to the border region between Pakistan and Afghanistan, where Osama bin Laden is believed to be hiding.

It was only when the arrests were made in Pakistan, the the order was sent to those in England to put the plot in motion.

From Atlanta to Albuquerque, N.M., airport maintenance crews were ordered to dump any confiscated items along with the rest of the garbage.

"They seal it, they do not touch it, they dispose of it as they would other garbage," said Daniel Jiron, spokesman for the Albuquerque International Sunport.

All the waste irked some passengers.

"I know they have to do this, but I think they went overboard," said Terry Asbury, an Ohio resident who flew from Albuquerque to Cincinnati on Thursday and had to throw out her cosmetics. "I literally lost about $50 or $60 worth of things we were told to throw out."

Britain was put on "critical" alert - the highest level of terror warning - up from "severe", early on Thursday morning as police launched raids in London, Birmingham and High Wycombe, Bucks.

Mr Reid told the BBC News 24 Sunday programme today: "Even if on the basis of our intelligence ... we reduced from 'critical' to 'severe' ... it is highly likely there will be another terrorist attempt and that is one thing of which we can be sure."

Responding to a report in The Observer that "up to two dozen" terror probes were operating across the country, the Home Secretary said that there could also be unknown threats.

"I'm not going to confirm an exact number, but I wouldn't deny that that would indicate the number of major conspiracies that we are trying to look at," he said. "There would be more which are not at the centre of our considerations and there may be more that we don't know about at all."

Asked how many terror plots capable of causing major loss of life had been foiled since July 7 last year, he said: "I can tell you that at least four major plots have been thwarted."

He also warned that despite the police operation, members of the alleged terror ring could still be "out there".

"We believe it was a major, major plot," he said. "We believe ... that we have the main targets from that particular surveillance and plot, as I said there are still people out there who would carry out such attacks."

The Blairs will be guests of Sir Cliff Richard at his Barbados villa, where they've stayed every year since 2002.

But as he relaxes in his garish beachwear the PM should be worried... he could get stuck in the Bermuda Triangle.

'Worst attacks yet'

"The terrorists attempt to bring down airplanes full of innocent men, women, and children," Mr Bush said.

"They kill civilians and American servicemen in Iraq and Afghanistan, and they deliberately hide behind civilians in Lebanon. They are seeking to spread their totalitarian ideology."

Mr Bush said that the alleged terror plot, which UK intelligence services claim involved a plan to destroy US-bound passenger planes using liquid explosives smuggled in drinks bottles, was "further evidence that the terrorists we face are sophisticated, and constantly changing their tactics".

US officials say that if the plan had not been foiled, the subsequent attacks would have been the worst since those on Washington and New York on 11 September 2001.

Since the 2001 attacks, Mr Bush has said that the US is engaged in a global war on terror.

He says that as well as intelligence efforts to foil terror plots against US civilians, the ongoing military campaigns in Afghanistan and Iraq are part of that same battle, as is Israel's conflict with Lebanon.

It is understood that Britain asked the US to avoid making any such assertion, but diplomats believe that the request was ignored by Michael Chertoff, the Homeland Security chief. There is suspicion that the speed with which the US linked al-Qaeda to the plot was motivated by political considerations because, before the November mid-term Congressional elections, Republicans are keen to stem voter anger against the Iraq war by focusing on national security.

One senior UK source said that with the fifth anniversary of the September 11 attacks approaching, "al-Qaeda is a term which is understood by swing voters". He added: "We regard this as simplistic."

On Thursday morning Mr Chertoff, the Homeland Security Secretary, used a press conference to point the finger of blame directly at al-Qaeda. Initially, he picked his words carefully, saying that the sophistication of the operation was "suggestive of alQaeda", while acknowledging this was a "sensitive area for the British legal system".

He later said that the plan was reminiscent of that "hatched by Khalid Sheikh Mohammed [the al-Qaeda mastermind behind 9/11] in the 1990s" to blow up aircraft travelling over the Pacific.

American television stations have been broadcasting a tape made by bin Laden this year in which he gave a warning that preparations were under way for another attack "in the heart of your land".

The remarks in the US were in sharp contrast to statements by the Home Office and the Metropolitan Police.

"We don't deal with hypothesis," said a Met spokeswoman, "and we don't discuss matters of intelligence - we never have and never will."

British authorities arrested two dozen suspects on Thursday for allegedly plotting to use liquid explosives to blow up airliners flying from Britain to the United States.

The arrests prompted the United States to raise its terror alert to the highest level ever and prompted airports to ban passengers from taking liquids, gels and creams on planes.

Bush, who returns to Washington on Sunday after a 10-day working vacation at his ranch, urged air travelers to be patient with the stricter security measures.

"The inconveniences you will face are for your protection and they will give us time to adjust our screening procedures to meet the current threat," he said.

Democrats on Friday accused Vice President Dick Cheney of trying to use this week's arrests in Britain to Republican advantage in November congressional elections, which will determine whether Democrats or Republicans control the U.S. Congress.

'AL QAEDA TYPES'

Cheney said on Wednesday the Democrats' defeat of Connecticut Democratic Sen. Joe Lieberman in the state's primary on Tuesday because of his support of the Iraq war could embolden "al Qaeda types."

Senate Democratic leader Harry Reid of Nevada said in a statement on Friday: "Once again, GOP (Republican) leaders are using terrorism and our national security as a political wedge issue. It is disgusting -- but not surprising."

Bush said the suspected plot in Britain "reminds us of a hard fact: The terrorists have to succeed only once to achieve their goal of mass murder, while we have to succeed every time to stop them."

"Unfortunately, some have suggested recently that the terrorist threat is being used for partisan political advantage. We can have legitimate disagreements about the best way to fight the terrorists, yet there should be no disagreement about the dangers we face," he said.

Democrats in their weekly radio address charged Bush has shortchanged domestic security needs and the war on terror, and they blamed him for bungling the Iraq war.

Sen. Mark Pryor of Arkansas said the administration's "poor management" in Iraq "has created a rallying cry for international terrorists" and "diverted our focus, our military and more than $300 billion from the war on terrorism."

Pryor said U.S. ports, borders and chemical plants remain unsecured, emergency personnel lack critical resources and the military, including the National Guard, was stretched.

"It's time for Washington to be tough and smart about the threats we face," he said. "Americans deserve real security, not just leaders who talk tough but fail to deliver."

The discovery prompted fears that there were fanatical mothers in secret al-Qaeda cells in Britain ready to become suicide bombers - and to die with their tots in their arms.

And it emerged as the reason why women at airports were ordered to drink from their babies' bottles before being allowed to board flights during last week's massive alert.

One senior Government security adviser warned of a race against time to identify individuals who might pose a threat.

The adviser said: "It may be beyond belief, but we are convinced that there are now women in Britain who are prepared to die with their babies for their twisted cause. They are ruthless, single-minded and totally committed."

The nightmare is that mums carrying tiny tots would provide "very good cover" and not raise suspicions among even the most alert security guards.

The threat was identified along with an additional warning that as many as two dozen terror cells may still be active in Britain.

The source added: "We believe all the known players involved in last week's plot have been detained. Our biggest concern now is all the unknown players who may be out there.

"And that includes mothers who are ready and willing to see their little ones die. It is a race against time." Women around the world have carried out suicide attacks in the past.

Two female Chechen terrorists blew themselves up on separate flights in Russia two years ago.

An intelligence source said: "Al-Qaeda specialises in attempting the unexpected. What could be more unexpected in Western eyes than women willing to die with their babies?"

MI5 chiefs are jubilant over cracking the plan to disguise bombs as soft drinks or baby bottles - but warned that other forms of transport are massively at risk from suicide attacks.

The Government source said: "There are dozens of other outrages being plotted on all forms of transport because they offer the highest potential body count."

Yesterday it emerged that among those detained in last week's raids is believed to be a key al-Qaeda figure.

A senior Government official said: "He is not al-Qaeda's Mr Big in Britain but he is certainly a very important player."

It is now almost certain that a dummy run for an attack had been planned by terrorists for a flight over the weekend.

"We now think that was on," said the source.

Last night 23 people arrested in raids in London, Birmingham and High Wycombe, Bucks, remained in police custody.

Twenty-two can be detained until Wednesday and a decision on another individual has been adjourned until today.

Reid said al-Qaeda had first attempted to attack Britain in 2000, and he suggested that more than 20 terrorist conspiracies are currently being investigated in the country.

Responding to a question about a report in the British press that there were "two dozen" terror cells under investigation, Reid said: "I'm not going to confirm an exact number, but I wouldn't deny that that would indicate the number of major conspiracies that we are trying to look at. There would be more which are not at the center of our considerations, and there may be more that we don't know about at all."

At London's Heathrow Airport on Sunday, security officials hand-searched every passenger, which created four times the normal workload and caused lines of passengers to stretch out of the terminal. Airlines operating from Heathrow canceled about 130 flights, people were prohibited from bringing any hand luggage onto airplanes, and some flights left without passengers who couldn't arrive in time, said British Airports Authority spokesman Duncan Bonfield.

"These are unprecedented circumstances: four times the volume of searches in the busiest international airport in the world," Bonfield said. "There has been a significant disruption, but I think people do understand that their safety and security is paramount.

"As yet we've had no indication whatsoever from the government about how long it's going to go on," he added.

One of the airlines operating in Britain, Ryanair, criticized the government's "heavy-handed" security measures that disrupted the travels of thousands of British passengers. The airline said in a statement that it was in favor of "all sensible and effective security measures" but that they should be limited so that fewer people are searched.

"If the main U.K. airlines are forced to continue to cancel flights because the airports cannot meet these security requirements then the extremists will have succeeded," the company said.

Ryanair's chief executive, Michael O'Leary, said his airline cannot cope with the security measures and asked the British government for security personnel to help with the searches.

"If the British government is serious about defeating terrorism and not allowing the terrorists to disrupt normal, everyday British life, then it must provide the additional security staffing -- either police or army reserve personnel -- immediately to prevent London's main airports from grinding to a halt over the coming days," O'Leary said, according to wire service reports.

Airport officials expected the problems to continue into the week, and British Airways on Sunday was planning to cancel 20 percent of its Monday flights from Heathrow to comply with a British Airports Authority directive.

[Late Sunday, U.S. Homeland Security cut the threat level for U.K-U.S. flights to orange, or high, from red, or severe, the Reuters news agency reported.]

Neither the TSA nor Suspect Detection Systems Ltd., the Israeli company, will discuss the Knoxville trial, whose primary goal was to uncover the designated bad guys, not to identify threats among real travelers. They won't even say what questions were asked of travelers, though the system is generally designed to measure physical responses to hot-button questions like "Are you planning to immigrate illegally?" or "Are you smuggling drugs."

The test alone signals a push for new ways to combat terrorists using technology. Authorities are convinced that beyond hunting for weapons and dangerous liquids brought on board airliners, the battle for security lies in identifying dangerous passengers.

The method isn't intended to catch specific lies, says Shabtai Shoval, chief executive of Suspect Detection Systems, the start-up business behind the technology dubbed Cogito. "What we are looking for are patterns of behavior that indicate something all terrorists have: the fear of being caught," he says.

Security specialists say such technology can enhance, but not replace, existing detection machines and procedures. Some independent experts who are familiar with Mr. Shoval's product say that while his technology isn't yet mature, it has potential. "You can't replicate the Israeli system exactly, but if you can incorporate its philosophy, this technology can be one element of a better solution," says Doron Bergerbest-Eilon, chief executive of Asero Worldwide consulting firm and a former senior official in Israel's security service.

To date, the TSA has more confidence in people than machines to detect suspicious behavior. A small program now is using screening officers to watch travelers for suspicious behavior. "It may be the only thing I know of that favors the human solution instead of technology," says TSA chief Kip Hawley.

The people-based program -- called Screening Passengers by Observation Technique, or SPOT -- began undergoing tests at Boston's Logan Airport after 9/11 and has expanded to about a dozen airports. Trained teams watch travelers in security lines and elsewhere. They look for obvious things like someone wearing a heavy coat on a hot day, but also for subtle signs like vocal timbre, gestures and tiny facial movements that indicate someone is trying to disguise an emotion.

TSA officers observe passengers while consulting a list of more than 30 questionable behaviors, each of which has a numerical score. If someone scores high enough, an officer approaches the person and asks a few questions.

"All you know is there's an emotion being concealed. You have to find out why the emotion is occurring," says Paul Ekman, a San Francisco psychologist who pioneered work on facial expressions and is informally advising the TSA. "You can find out very quickly."

More than 80% of those approached are quickly dismissed, he says. The explanations for hiding emotions often are innocent: A traveler might be stressed out from work, worried about missing a flight or sad because a relative just died. If suspicions remain, the traveler is interviewed at greater length by a screener with more specialized training. SPOT teams have identified about 100 people who were trying to smuggle drugs, use fake IDs and commit other crimes, but not terrorist acts.

The TSA says that, because the program is based on human behavior, not attributes, it isn't vulnerable to racial profiling. Critics worry it still could run afoul of civil rights. "Our concern is that giving TSA screeners this kind of responsibility and discretion can result in their making decisions not based on solid criteria but on impermissible characteristics such as race," says Gregory T. Nojeim, associate director of the American Civil Liberties Union's Washington legislative office.

Mr. Shoval, the Israeli entrepreneur, believes technology-based screening is the key to rolling out behavior-recognition techniques in the U.S. With experience in counter-terrorism service and the high-technology industry, Mr. Shoval developed his Cogito device with leading former Israeli intelligence officials, polygraph experts and computer-science academics.

Here is the Cogito concept: A passenger enters the booth, swipes his passport and responds in his choice of language to 15 to 20 questions generated by factors such as the location, and personal attributes like nationality, gender and age. The process takes as much as five minutes, after which the passenger is either cleared or interviewed further by a security officer.

At the heart of the system is proprietary software that draws on Israel's extensive field experience with suicide bombers and security-related interrogations. The system aims to test the responses to words, in many languages, that trigger psycho-physiological responses among people with terrorist intent.

The technology isn't geared toward detecting general nervousness: Mr. Shoval says terrorists often are trained to be cool and to conceal stress. Unlike a standard lie detector, the technology analyzes a person's answers not only in relation to his other responses but also those of a broader peer group determined by a range of security considerations. "We can recognize patterns for people with hostile agendas based on research with Palestinians, Israelis, Americans and other nationalities in Israel," Mr. Shoval says. "We haven't tried it with Chinese or Iraqis yet." In theory, the Cogito machine could be customized for specific cultures, and questions could be tailored to intelligence about a specific threat.

The biggest challenge in commercializing Cogito is reducing false results that either implicate innocent travelers or let bad guys slip through. Mr. Shoval's company has conducted about 10 trials in Israel, including tests in which control groups were given terrorist missions and tried to beat the system. In the latest Israeli trial, the system caught 85% of the role-acting terrorists, meaning that 15% got through, and incorrectly identified 8% of innocent travelers as potential threats, according to corporate marketing materials.

The company's goal is to prove it can catch at least 90% of potential saboteurs -- a 10% false-negative rate -- while inconveniencing just 4% of innocent travelers.

Mr. Shoval won a contract for the Knoxville trial in a competitive process. Next year, Israeli authorities plan to test Cogito at the country's main international airport and at checkpoints between Israel and the West Bank, where the goal will be to catch genuine security threats while testing the logistics of using the system more broadly. The latest prototype costs about $200,000 a machine.

Even though his expertise is in human observation, U.S. behavior-recognition expert Dr. Ekman says projects like Cogito deserve a shot. He expects technology to advance even further, to devices like lasers that measure people's vital signs from a distance. Within a year, he predicts, such technology will be able to tell whether someone's "blood pressure or heart rate is significantly higher than the last 10 people" who entered an airport.

Write to Jonathan Karp at jonathan.karp@wsj.com and Laura Meckler at laura.meckler@wsj.com

Finally, he came to a shocking conclusion that runs counter to the accepted history of America's darkest day: The towers, he believes, "were brought down by planted explosives."

He's not alone.

Henry and several other Bay State residents are members of Scholars for 9/11 Truth, a controversial group that claims elements of the U.S. government, not Osama bin Laden, masterminded the deadly attacks that killed almost 3,000 Americans.

Members of the group, including about 80 professors nationwide, generally believe the attacks were designed around building support for an aggressive U.S. strategy in the Middle East.

Members point to a string of what they describe as discrepancies in the accepted history of the attacks, including continuing uncertainty about why a third World Trade Center tower, known as Building 7, collapsed without being struck by a plane.

"There is something hugely wrong with the official story," said Gwendolyn Atwood, 45, of Lincoln, a clinical psychologist trained at Harvard University and a group member.

The group's theories collide with the findings of the 9/11 Commission and an exhaustive investigation by the National Institute of Standards and Technology, a government agency, launched to determine the cause of the buildings' collapse.

Fires resulting from the impact of the fuel-laden airliners destroyed the twin towers, according to reports by the NIST, which assigned 200 employees to the two-year investigation.

The agency interviewed more than 1,000 people near the scene of the attack or who helped design the buildings, analyzed 236 pieces of metal from the wreckage and studied 150 hours of video and almost 7,000 photographs capturing the collisions and collapses.

The agency's final report rejects "alternative hypotheses suggesting the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001."

Many people, however, are not convinced.

A poll released this week by Scripps Howard News Service found that 36 percent of Americans believe "people in the federal government either assisted in the 9/11 attacks or took no action to stop the attacks because they wanted to United States to go to war in the Middle