The anthrax attacks caused panic in the US shortly after 9/11 [EPA]

At best, you can take the FBI at its word, and one of the seminal events in the lead up to the Iraq war - that contributed to the heightened sense of threat and national hysteria that was used to justify the invasion - was the work of one unstable scientist employed at a US military laboratory.

At worst, the FBI is trying to cover up its mismanagement of the investigation and the aggressive tactics used, and whoever was responsible for the anthrax that was mailed to members of the US congress and the media in late 2001, which ended up killing five people, will now remain a mystery.

The source of the anthrax has been established, traced to a batch grown at the Fort Detrick laboratory in the US state of Maryland.

Contrary to public assumptions at the time, neither Iraq nor Saddam Hussein was ever involved.

The FBI has also proved, beyond reasonable doubt, that Bruce Ivins, the scientist who committed suicide in late July - apparently after hearing that he would be indicted for mass murder - was seriously weird.

A 'patsy'?

The clear imperative to justify a seven-year investigation into the worst act so far of bio-terror in the US inevitably raises the spectre of the "patsy", the phrase made famous by Lee Harvey Oswald after his arrest for the assassination of John F Kennedy, the former US president.

Former researcher Steven Hatfill was wrongly accused of the attacks [EPA]

One of the reasons is that Oswald, like Ivins, never lived to answer the charges.

The anthrax case is further complicated by the fact that the FBI had first set its sights on a different suspect.

In the spirit of post 9/11, when all the rules were suspended, and torture and throwing people in jail without trial were permitted, the FBI was unscrupulous in the pursuit of its prey.

That ended up costing US taxpayers more than $5m, as Steven Jay Hatfill, who also worked at Fort Detrick, sued the government for making his life hell and destroying his reputation.

Ruthless tactics

The tactics used against the subsequent suspect, Ivins, were particularly ruthless.

The anthrax investigation was one of the most costly in US history [AFP]

FBI officials are alleged to have confronted him and his family in a shopping mall, brandishing photographs of the anthrax victims and calling him a murderer.

It would be enough to drive anyone, let alone someone with a tenuous grasp on sanity, over the edge.

In Ivins's case it succeeded. We are now all but invited to read his suicide as a confession.

Either that, or the FBI has some explaining to do.

Theories continue

It is a lot easier to convict dead men. In Oswald's case there was a previous assassination attempt, the subsequent murder of a policeman, and a dramatic arrest in a movie theatre.

Yet there persists a general bewilderment about the Kennedy assassination.

And in the Ivins case there is nothing approaching conclusive evidence, other than the absolute necessity for the FBI to draw a costly and controversial investigation to a successful conclusion.

The September 11 attacks themselves overshadow the anthrax letters, and though they were less televisual, with a much lower body count, there is no underestimating the effect that they had on the mood of the country at a critical moment in its history.

The explanation now on offer for this strange and sinister event is far less satisfactory than the account of what happened in Dallas in 1963.

It is now destined to recede to that "grassy knoll" in the public imagination, and be the fruit of conspiracy theories for years to come.