Iranian Nuclear plant
© Hamid Foroutan / APIran's heavy water nuclear facilities near the central city of Arak
More than three years ago, the CIA dispatched a stealth surveillance drone into the skies over Iran.

The bat-winged aircraft penetrated more than 600 miles inside the country, captured images of Iran's secret nuclear facility at Qom and then flew home. All the while, analysts at the CIA and other agencies watched carefully for any sign that the craft, dubbed the RQ-170 Sentinel, had been detected by Tehran's air defenses on its maiden voyage.

"There was never even a ripple," said a former senior U.S. intelligence official involved in the previously undisclosed mission.

CIA stealth drones scoured dozens of sites throughout Iran, making hundreds of passes over suspicious facilities, before a version of the RQ-170 crashed inside Iran's borders in December. The surveillance has been part of what current and former U.S. officials describe as an intelligence surge that is aimed at Iran's nuclear program and that has been gaining momentum since the final years of George W. Bush's administration.

The effort has included ramped-up eavesdropping by the National Security Agency, formation of an Iran task force among satellite-imagery analysts and an expanded network of spies, current and former U.S. officials said.

At a time of renewed debate over whether stopping Iran might require military strikes, the expanded intelligence collection has reinforced the view within the White House that it will have early warning of any move by Iran to assemble a nuclear bomb, officials said.

"There is confidence that we would see activity indicating that a decision had been made," said a senior U.S. official involved in high-level discussions about Iran policy. "Across the board, our access has been significantly improved."

The expanded intelligence effort has coincided with a covert campaign by the CIA and other agencies to sabotage Iran's nuclear program and has enabled an escalation in the use of targeted economic sanctions by the United States and its allies to weaken Iran's resolve.

The Obama administration has cited new intelligence reports in arguing against a preemptive military strike by Israel against Iranian nuclear facilities.

Israeli officials have pushed for a more aggressive response to Iran's nuclear activities, arguing that Iran is nearing what some officials have called a "zone of immunity," in which Iran can quickly complete the final steps toward becoming a nuclear power inside heavily fortified bunkers protected from Israeli airstrikes.

White House officials contend that Iran's leaders have not decided to build a nuclear weapon, and they say it would take Iran at least a year to do so if it were to launch a crash program now.

"Even in the absolute worst case - six months - there is time for the president to have options," said the senior U.S. official, one of seven current or former advisers on security policy who agreed to discuss U.S. options on Iran on the condition of anonymity.

The improved intelligence also strengthens the administration's bargaining position ahead of nuclear talks with Iran, tentatively scheduled for Friday. The United States and five other countries - Russia, China, Britain, France and Germany - are expected to press Iran to accept curbs on its nuclear program that would make it far more difficult for the country to build a nuclear weapon. A key demand, Western diplomats say, is for Iran to halt production at its uranium enrichment plant at Qom, which was built in mountain tunnels beyond the reach of all but the most advanced bombs and missiles. In return for such a concession, Iran could be allowed to keep some semblance of a commercial nuclear power program under heavy international oversight, diplomats say. It is unclear, however, whether Iran would agree to restrictions on its program. In recent days, Iran has refused even to commit to a venue for the talks.

The CIA declined to comment on the nature of its operations against Iran. Officials familiar with the operations, however, acknowledged that there had been some setbacks and conceded that aspects of Iran's nuclear decision-making remain opaque, including the calculations made by the Islamic republic's senior political and clerical leadership.

Iranian officials insist publicly that the program is for peaceful energy production. But experts skeptical of that explanation warn that Iran may become more adept at hiding parts of its nuclear program, particularly if it succeeds in building more powerful centrifuges that can enrich uranium in smaller, dispersed facilities.

"They have been taken off-guard in the past, and now they do their best to conceal," said Olli Heinonen, who formerly directed nuclear inspections inside Iran for the International Atomic Energy Agency. While Western spy agencies have been successful of late, he said, "they are shooting at a moving target."

The still-fresh sting of Iraq

There is also the chastening experience of Iraq. A decade ago, analysts at the CIA and other agencies were confident that Iraq had stockpiles of banned weapons, including the components of a nuclear weapons program. A costly U.S. invasion and futile search for those stockpiles proved them wrong.

The sting of that intelligence failure was still fresh when U.S. spy agencies came under pressure to ramp up collection efforts against Iran. By 2006, U.S. intelligence officials and top Bush advisers had become alarmed by deep gaps in U.S. knowledge of Iran's nuclear efforts and ambitions.

Michael V. Hayden, then the new CIA director, recalled a White House briefing in which Bush became visibly agitated.

At the time, Iran was rapidly expanding its stockpile of enriched uranium at its main Natanz facility while working on what was then a secret site at Qom. American officials feared that Iran might surprise the world with a nuclear weapons test that would leave U.S. leaders with two highly unpalatable options: Attack Iran or accept the emergence of a new nuclear power in the Middle East.

At one point, Bush turned to Hayden and said, "I don't want any U.S. president to be faced with only two choices when it comes to Iran," according to Hayden. Efforts to reach Bush for comment were not successful.

The meeting became the impetus for overhauling the CIA's approach to a country considered one of its hardest targets. The agency's Iran experts and operatives were moved from its Near East Division to a group focused exclusively on Iran, much as the CIA had formed its Counterterrorism Center 20 years earlier.

"We put the best people on the job and put the most talented people in charge," Hayden said. "Then we said, 'Tell us what you need to get the job done.' "

Known internally as "Persia House," the Iran Operations Division was set up in the agency's Old Headquarters Building. Over time, it swelled from several dozen analysts and officers to several hundred. The division is now headed by a veteran case officer who previously served as CIA station chief in Islamabad, Pakistan.

"It got a robust budget," said a former senior CIA official who worked in the Near East Division at the time. The Iran division's emphasis was "getting people overseas in front of people they needed to be in front of - there are a lot of places to meet Iranians outside Iran."

The division began assembling an informant network that stretched from the Middle East to South America, where Iran's security services have a long-standing presence. The CIA also exploited the massive U.S. military presence in Afghanistan and Iraq to mount espionage operations against the country sandwiched between those war zones.

Limited damage

One of those operations was exposed last year, when an RQ-170, flown from an airstrip in Afghanistan, crashed inside Iran. Officials in Tehran have triumphantly claimed credit for bringing the stealth drone down and have released pictures showing the drone apparently patched up after the crash. U.S. officials say a technical failure caused the crash.

The former intelligence official familiar with the beginnings of the stealth drone missions said that there had been pointed debate before deploying the first aircraft over whether it should be equipped with a so-called self-destruction package, which could blow an RQ-170 to bits if it flew off course.

The director of national intelligence at the time, Michael McConnell, was among the high-ranking officials who pushed to have the package installed. But the CIA's engineering team balked, saying it would add too much weight to the delicately balanced frame.

Despite the setback, U.S. officials said that some surveillance flights continue and that the damage to American espionage capacity overall has been limited.

That is partly because the drone flights were only a small part of a broad espionage campaign involving the NSA, which intercepts ­e-mail and electronic communications, as well as the National Geospatial-Intelligence Agency, which scours satellite imagery and was the first to spot the uranium enrichment plant at Qom.

The CIA's expanded efforts continued under director Leon E. Panetta, who built partnerships with allied intelligence services in the region capable of recruiting operatives for missions inside Iran, former intelligence officials said.

The agency has encountered problems. Shahram Amiri, an Iranian defector and scientist in the country's nuclear program, had been given $5 million by the CIA and relocated to Tucson. But in 2010, he abandoned his American life and returned to Tehran - where he had a young son - giving Iranian officials not only a propaganda victory but probably information on what his CIA debriefers were most desperate to learn.

U.S. officials said Amiri had been handled by the CIA's Counter­proliferation Division after he approached U.S. officials in Vienna and volunteered to spy. That division continues to handle scientists and technical experts connected to Iran's program, while Persia House focuses on leadership figures and the nation's sprawling military and security services, including the Republican Guard Corps.

"The real damage was image - we looked like the Keystone Kops," said a former senior CIA official of Amiri's return to Iran. "In terms of actual damage - no, we collected all kinds of great stuff."

The expanded espionage effort has confirmed the consensus view expressed by the U.S. intelligence community in a controversial estimate released publicly in 2007. That estimate concluded that while Iran remains resolutely committed to assembling key building blocks for a nuclear weapons program, particularly enriched uranium, the nation's leaders have opted for now against taking the crucial final step: designing a nuclear warhead.

"It isn't the absence of evidence, it's the evidence of an absence," said one former intelligence official briefed on the findings. "Certain things are not being done."

Staff writer Julie Tate contributed to this report.