© Getty ImagesSuicided: Dr David Kelly
The day Dr David Kelly took a short walk to his death in the Oxfordshire countryside, an unopened letter lay on the desk of his book-lined study.

Sent from the heart of the British Government, the pages were marked 'personal' and threatened the world-renowned microbiologist with the sack if he ever publicly opened his mouth again.

The letter remained unopened for the seven days during the drama that would pitch Dr Kelly into the spotlight and end in his death at just 59.

No one has ever explained why the eminent scientist and UN weapons inspector did not open the letter, but everyone close to him is convinced he knew its contents.

It was designed to silence him because his Ministry of Defence bosses had discovered that not only was he secretly talking to journalists, but was also preparing to write an explosive book about his work.

It was six years ago tomorrow, on July 17, 2003, that Dr Kelly was found dead under a tree on Harrowdown Hill half a mile from his family home in Southmoor. His fate has become one of the most contentious issues of recent political history and has raised profound questions about the moral integrity of the New Labour government.

The former grammar school boy had celebrated his 36th wedding anniversary just a few days before.

The questions of why and how he died - and if he was murdered - have never gone away.

Dr Kelly had examined the Government's 'sexed up dossier' which declared that Saddam Hussein had weapons of mass destruction which could be activated in just 45 minutes. The claim was used by Tony Blair in 2002 as the central justification for the Iraq war.

When Dr Kelly secretly revealed his doubts about the dossier to BBC reporters, all hell broke loose.

After he was unmasked as the BBC mole, he was marched before the television cameras of a House of Commons committee and, later, taken away to a safe house to be interviewed by the British intelligence services.

In one final phone conversation he told a caller he wouldn't be surprised 'if my body was found in the woods'.

And so it was to be. The official inquiry into his death later decided that he committed suicide - by slashing his wrist and consuming a cocktail of painkillers.

But this week, 13 respected doctors declared that it was medically impossible for Dr Kelly to have died in this manner. They are mounting a legal battle to overturn the suicide verdict.

A new film, Anthrax War, to be released in London this weekend, also asserts that Dr Kelly had spent hours writing a tell-all book which would violate the Official Secrets Act by exposing Britain's dubious authority for toppling Saddam Hussein.

The film, directed by New York-based documentary maker Bob Coen, states that Dr Kelly, head of biological defence at the Government's secretive military research establishment of Porton Down, Wiltshire, was the brain behind much of the West's germ warfare programmes. Quite simply, the film says, Dr Kelly 'knew too much'.

In further unsubstantiated and hard-to-believe claims, the film alleges he may have been embroiled in apartheid South Africa's Project Coast programme to develop an ethnic germ weapon programme to target the black population.

Coen also says Dr Kelly had links to illegal human experiments on British servicemen at Porton Down, which sparked the largest ever investigation by Wiltshire Police.

Officers recommended charges against some scientists at the germ warfare establishment - but dropped the idea just days after Dr Kelly was found dead.

Whatever the veracity of all this, the film's central thrust - that he was writing a sensational book - has been confirmed by Gordon Thomas, a British intelligence expert, who had met Dr Kelly.

Thomas told me: 'I visited Dr Kelly as part of research into a book I was writing. But he told me that he was writing his own book, which intended to show that Tony Blair had lied about his reasons for going to war with Iraq.

He had told the Prime Minister categorically that there were no weapons of mass destruction.'

Thomas, in his own book, states: 'Dr Kelly was not a man given to exaggeration or showing off; he was the absolute expert in his field and if he said there no weapons of mass destruction, then there were none.

'I told Dr Kelly he would never be allowed to publish his book in Britain. I told him he would put himself into immense danger.

His plan was to resign from Porton Down and move with his wife to the United States where he could make more money from his revelations.'

Can this possibly be true? Certainly, Dr Kelly lived a double life. At home in Oxfordshire with wife Janice, he was the perfect husband.

The couple would have supper together in the garden after he had spent hours in what she called 'his secret world' - the book-lined study off the hallway.

Here, computers linked him to the Britain's intelligence services MI5 and MI6, GCHQ, the Ministry of Defence (MoD), the Foreign Office and foreign spy agencies - including Israel's notorious Mossad (for whom he had worked since 1995 as an advisor with the blessing of Whitehall).

Although he had an office in London - Room 2/35 in the MoD's Proliferation and Arms Control Secretariat - and another at Porton Down, Dr Kelly kept his secret data at home, including tens of thousands of documents and photographs; some show human victims of anthrax poisoning, as well as animal 'guinea pigs' poisoned with anthrax and other germs in labs across the world. For a man who was not a spy, it was an impressive collection.

From all round the globe he was consulted on biological weaponry, in particular the use of anthrax.

'You couldn't commit suicide like that'

Thomas takes up the story. 'Each intelligence organisation had installed its own computer for Dr Kelly to use on its behalf and to exchange encrypted messages. But Dr Kelly always said that most important information was filed in his head.'

However, perhaps fatally for Dr Kelly, his book was not only in his head. It was on hard-disk in one of his computers, which have all been seized by MI5 and are unlikely ever to see the light of day.

By any standards, the book would have been hugely contentious. In addition to Tony Blair and the British Government, there are any number of foreign intelligence agencies who would not want a public airing of the explosive information which they shared with Dr Kelly over the years.

His book was also expected to expose a black market trade in anthrax which was being exploited, and thus condoned, by many governments.

But it has now come to light that there may be another compelling reason why Dr Kelly might have been murdered.

Amazingly, 12 other well-known micro-biologists linked with germ warfare research have died in the past decade, five of them Russians investigating claims that the Israelis were working on viruses to target Arabs.

The Russian plane in which they were travelling from Tel Aviv to Siberia was shot down on October 2001 over the Black Sea by an 'off-course' Ukrainian surface-to-air missile.

Dr Kelly knew the victims and asked MI6 to find out more details. However, they drew a blank.

Five weeks later, Dr Benito Que, a cell biologist known to Dr Kelly, was found in a coma near his Miami laboratory.

The infectious diseases expert had been investigating how a virus like HIV could be genetically engineered into a biological weapon.

Dr Que, 52, was found unconscious outside in the car park of his lab and died in hospital. Officially, he suffered a heart attack - although his family say he was struck on the head. Police refused to re-open the case.

Ten days after Dr Que's death, another friend of Dr Kelly died. Dr Don Wiley, 57, one of America's foremost microbiologists, had a U.S. Government contract to create a vaccine against the killer Ebola fever and other so-called doomsday germs.

His rental car was found abandoned on a bridge across the Mississippi. The keys were in the ignition and the petrol tank full. There had been no crash, but Dr Wiley had disappeared.

The FBI visited Wiley's laboratory and removed most of his work. A month later his body was found 300 miles downstream, with evidence of severe head injuries. No forensic examination was performed and his death was ruled 'accidental'.

Little wonder, then, that Dr Kelly had begun talking about his body being 'found in the woods'.

And there is more. The most mysterious death of them all happened to Dr Vladimir Pasechnik - a Soviet defector Dr Kelly knew well.

The biochemist had left a drugs industry fair in Paris in 1989, just before the collapse of Communism, saying he wanted to buy souvenirs for family. Instead, he went to the British Embassy where he announced to a startled receptionist that he was a Russian scientist who wanted to defect.

Pasechnik was whisked secretly back to Britain, and Dr Kelly was brought in to verify his claims that the Soviets were adapting cruise missiles armed with germs to help spread killer diseases such as plague and smallpox.

As chief director of the Institute for Ultra-Pure Biological preparations in St Petersburg, Pasechnik had developed killer germs. 'I want the West to know of this. There must be a way to stop this madness,' he told Dr Kelly in a safe house.

Dr Kelly later told the author Gordon Thomas that he believed Pasechnik. 'I knew that he was telling the truth. There was no waffle. It was truly horrifying.'

The two scientists became friends. And soon Vladimir had set up the Regma Biotechnologies laboratory, near Porton Down. He seemed healthy when he left work on the night of November 21, 2001.

Returning home, the 64-year-old cooked supper and went to sleep. He was found dead in bed the next day.

Officially, the reason given was a stroke. However the Wiltshire police later said his demise was 'inexplicable'.

It is against this extraordinary background of highly suspicious deaths that Dr Kelly's own death occurred.

As we know, an inquest on his body was ruled out by Oxfordshire's coroner, a highly unusual move.

'Don't be surprised if my body is found.'

Instead, Tony Blair ordered an inquiry by Lord Hutton. It heard evidence from 74 witnesses and concluded that Dr Kelly killed himself by slashing the ulnar artery of his left wrist with a garden knife after swallowing painkillers - although none had been prescribed by his GP.

A detailed medical dossier by the 13 British doctors, however, rejects the Hutton conclusion on the grounds that a cut to the small ulnar artery is not deadly.

The dossier is being used by lawyers to demand a proper inquest and the release of Dr Kelly's autopsy report, which has never been made public. Their evidence will be sent to Sir John Chilcot's forthcoming Iraq War inquiry.

One of the doctors, David Halpin, former consultant in trauma at Torbay Hospital, Devon, told me: ' Arteries in the wrist are of matchstick thickness and severing them does not lead to life-threatening blood loss.'

He and the other doctors say: 'To die from haemorrhage, Dr Kelly would have had to lose about five pints of blood.

It is unlikely from his stated injury that he would have lost more than a pint.' A lack of blood at the death scene was also confirmed by the search team who found Dr Kelly and the paramedics who tried to treat him.

One of the country's most respected vascular surgeons, Martin Birnstingl, also says that it would be virtually impossible for Dr Kelly to have died by severing the ulnar artery on the little finger side of his inner wrist.

'I have never, in my experience, heard of a case where someone has died after cutting their ulnar artery.

The minute the blood pressure falls, after a few minutes, this artery would stop bleeding. It would spray blood about and make a mess but it would soon stop.'

He believes that if Dr Kelly was really intent on suicide he would have cut the artery in his groin.

Dr Kelly was also right-handed - which meant he would have to slash awkwardly from left to right on his opposite wrist to have cut into the ulnar artery to any depth.

And what of the tablets? The almost empty packet of Co-Proxamol found by the dead scientist's side suggested he had taken 29.

But he had vomited and only a fragment of one remained in his stomach. The level of painkillers in his blood was a third of what is required to cause death.

As David Halpin says: 'The idea that a man like Dr Kelly would choose to end his life like that is preposterous. This was a scientist, an expert on drugs.'

So what really happened to Dr Kelly? The gardening knife that Lord Hutton said killed him was blunt and - although the scientist was not wearing gloves - had no fingerprints on it.

Which brings us back to that unopened letter found on Dr Kelly's desk, which had been sent to him at his home by MoD bosses and signed by Richard Hatfield, the ministry's personnel chief.

A whole series of experts died in strange ways

It emerged at the Hutton inquiry into Dr Kelly's death that it contained threats demanding his future silence.

At the time, Dr Kelly had received a number of warning phone calls at his home from the MoD about his indiscreet behaviour - and he will have been in no doubt that the official letter was written confirmation of these admonishments.

But he would not be put off. He saw his book as a guarantee of his financial future, which he often worried about.

On what he felt was a lowly £58,000 a year, the scientist fretted that his Government pension (based on his final salary) would not finance a decent retirement for him and his wife.

On the day he died, Janice has confirmed her husband was a distressed man. Dr Kelly lunched with her, before going out for a walk on Harrowdown Hill at 3.30pm.

It was a walk he made regularly at the same time of day - something anyone watching his movements would have been well aware of.

That day, events were already in motion elsewhere. An hour before, at 2.30pm, a senior policeman sat down at his computer at Thames Valley Police headquarters in Oxfordshire.

He began to create a restricted file on his secure computer. Across the top he typed a code name: Operation Mason. Although its contents have never been made public, it would detail the overnight search for Dr Kelly.

Incredibly, he created this file an hour before the scientist even left home.

After Dr Kelly's corpse was found at 8.30am by the volunteer searchers, the senior policeman made his last Operation Mason entry. It simply states: '9.00am. 18.07.03. Body recovered'.

Most intriguingly, at 8am, half an hour before Dr Kelly's body was discovered under the tree, three officers in dark suits from MI5's Technical Assessment Unit were at his house.

The computers and the hard-disk containing the 40,000 words of the explosive book were carried away. They have never been seen since.