David Kelly killed himself after a senior reporter betrayed him as the source for a story.
The betrayal broke the most sacred law of journalism, but Kelly's denial of involvement – a foolish lie – led directly to his death.
The exposure of that lie would have meant that his career would end in disgrace and he would retire a broken man in a marriage that had effectively run its course earlier.
At the height of the row between the BBC and the government in 2003, when BBC reporter Andrew Gilligan first made the broadcast alleging that the administration, and the Prime Minister, Tony Blair, had taken part in "sexing up" the intelligence dossiers that were to be a major justification for the second Iraq war, David Kelly was "outed" by the Ministry of Defence as Gilligan's source for the story.
At once, a titanic struggle took place between the BBC and government to protect their reputations. The BBC had to prove that Gilligan's report was wholly accurate; the government had to prove that it was untrue, maybe even fabricated. If Gilligan's story had been right, the Prime Minister would have been forced to resign. The stakes were very high.
Yet in one way, the BBC was fighting with a hand behind its back. Kevin Marsh, editor of the Today programme on which Gilligan made his remarks, said the piece was marred by flawed reporting, loose use of language, and lack of judgement in some of Gilligan's phraseology. To make matters worse, Gilligan had made notes on a personal organiser which never did retain a full and accurate record of the conversation with Kelly, and supporting hard copy notes that Gilligan says he made at the time were subsequently "lost".
Then, on 15 July, the Foreign Affairs Committee held a hearing at which Kelly gave evidence. Kelly was asked by the then Liberal Democrat MP David Chidgey whether he had briefed the Newsnight reporter Susan Watts about the government's intelligence dossiers on Iraq's alleged weapons of mass destruction. Chidgey then read to Kelly a very long quotation from the piece broadcast by Newsnight on 2 June.
Kelly replied he didn't recognise the words, became rather evasive, saying the words didn't sound like his, and finally specifically denied being the author of them. This was the lie that was to lead to his suicide.
I believe Kelly lied because he had been warned by his MoD bosses that if any other skeletons fell out of the cupboard involving him and unauthorised press briefings, his future would be in doubt. But he had already been betrayed: what the arms inspector never knew was that Andrew Gilligan, in a breach of journalistic ethics, had previously emailed Chidgey, suggesting that Kelly had indeed been the author of those words and the source for Watts's broadcast. Worse still, Kelly did not know when he gave evidence that Watts had, quite properly, recorded the interview with him for the purpose of record only. She had not warned Kelly.
But how did Gilligan know Kelly was the source? Watts didn't tell him. He says it was a lucky shot in the dark. But there is an alternative theory. Watts revealed Kelly's name only to her editor, George Entwistle. It is likely that Kelly's name reached top BBC bosses who needed to prove Kelly had briefed not only Gilligan but several BBC reporters. In turn, Gilligan, I believe, learned Kelly had been Watts's source. With this, Gilligan had the ammunition he needed to arm David Chidgey at the hearing.
Yet all this was unknown to Kelly on the morning of 17 July, his last day. Indeed, he was beginning to feel that he had probably got away with the lie. We know that, although he was tired, he was sufficiently cheerful to send several emails, all optimistic, to friends, looking forward to his scheduled return to Baghdad in a few days.
He had already been considering a new life. I know that he was planning to take a job at Stanford University, California, after his retirement a year later. This would have meant separation from his wife for at least a year. He had been partially dissuaded from this by a mutual friend, but the tensions in his private life had become as meaningful as his problems with Gilligan, Watts and his MoD bosses.
During that last morning, Kelly took several calls from Wing Commander John Clark who had become his MoD assistant-cum-minder during these difficult weeks. Some of the calls were routine, in connection with forthcoming parliamentary private questions about Kelly's journalistic contacts.
However, late in the morning, according to his wife, Kelly suddenly became withdrawn, silent, and thoughtful. With her keen sense of his unspoken moods, she felt so ill at the change that she went upstairs to throw up and then lie down. "I was physically sick because he looked so desperate ... I think he had a broken heart."
So what was the tipping point? Ironically, Andrew Gilligan appears to have the answer. Last week he wrote that Kelly had been caught out in an untruth "which, on the morning of his death, his bosses told him they would investigate". This is news.
If Gilligan is right, and I believe he is, then what changed Kelly's demeanour was a message he can only have received over the telephone. I believe that message could have been that the Susan Watts affair would shortly reveal him to have lied because the BBC had a tape recording of his conversation with Susan Watts.
If this message was passed to him – clearly not with malign intent – by someone in the MoD, it could have been what led to his suicide. For the first time, David Kelly was trapped.
The working-class lad from the Rhondda Valley who had fought his way to the top of his profession, relied on his integrity, honour and truthfulness. He could not afford the odium of exposure as a liar.
Rod Barton, an Australian arms inspector and close friend of Kelly's, told me: "David was jealous of his reputation and his dignity. He did not take criticism easily. One had to be very careful when expressing anything that could be construed as a negative view of his actions or opinions. He was exceptionally sensitive to even the slightest suggestion that he was not telling the truth."
The last time Wing Cdr Clark spoke to Kelly was at 2.53pm. Susan Watts's name came up. It was a significant discussion, for a decision had just been taken by the MoD on how to define Kelly's relationship with Watts, for the purposes of the formal replies to the parliamentary questions about Kelly's contacts with reporters. Her name, he was informed, was to be transferred from his list of generic or occasional contacts to more specific and detailed contacts. In other words, the net was closing in.
Only 10 minutes later, Clark phoned again. There was no reply. Kelly had gone off to kill himself. Clark phoned Janice Kelly. He was told David had gone for a walk. Clark asked Mrs Kelly to get David to call him on his return.
Then Clark did an odd thing. He called Kelly every 15 minutes on his mobile for the next hour and 50 minutes – seven to eight calls without a reply. Clark says he did this in case Kelly switched his mobile on. But one must wonder if Clark had inadvertently passed on to Kelly something so devastating, that in retrospect, he may have wished he hadn't told him, and maybe the urgency with which he tried, but failed, to call him back was to ensure David didn't do anything foolish. It was too late.
Tom Mangold is a former Panorama reporter and was a long-term friend of David Kelly