A top government scientist who helped to investigate America's deadly 2001 anthrax attacks has killed himself just as he was about to be charged in the case, in an extraordinary and unexpected twist to the biggest criminal investigation in US history.

Bruce Ivins, 62, who had worked for 18 years at the US Government's biodefence research laboratory in Fort Detrick, Maryland, died from an overdose of painkillers after being told that the Justice Department was about to charge him over the attacks, which brought fresh terror to the US days after the September 11 atrocity.

The death of Dr Ivins may put an end to one of the most baffling criminal investigations of modern times: the posting of weapons-grade anthrax spores in September and October 2001 that killed five people, sickened 17, closed Capitol Hill and crippled the US Postal Service.

Convinced at the time that it was another al-Qaeda plot, President Bush has since conceded that it was one of the lowest points of his White House tenure.

Dr Ivins, a microbiologist, helped the FBI to analyse materials recovered from one of the anthrax-tainted envelopes that was sent to the office of a senator in Washington.

He came under suspicion last year after the FBI put two new investigators on the case.

They reinterviewed dozens of scientists at the Fort Detrick laboratory after becoming convinced that it had been the source of the anthrax used in the attacks.

Until his death Dr Ivins had not been named publicly as a suspect in the seven-year investigation. His lawyers were informed that he was about to be charged and that prosecutors would be seeking the death penalty.

His suicide came after a government payment of $5.82 million (£3 million) last month to Steven Hatfill, another scientist at the Fort Detrick plant, whose life was left in ruins after being named a “person of interest” in 2002 by John Ashcroft, then the Attorney-General.

Mr Hatfill has always protested his innocence.

It was unclear what motive Dr Ivins would have had for the attacks. His brother, Tom, described him as “a man with no guts who liked people to bow down to him”. He said that they had not spoken for 20 years. Mr Ivins said: “He was a weak character but he liked to have people worship him.”

Spore-laden letters were posted on September 18 and October 9, 2001, to media organisations in New York and Florida and to the offices of Tom Daschle, then the Senate Democratic leader, and a colleague, Senator Patrick Leahy, of Vermont.

One of the five people killed was Bob Stevens, a British picture editor from Berkshire who worked in Boca Raton, Florida. Two postal workers, a New York hospital worker and an elderly woman in Connecticut also died. The Senate, House of Representatives and the Supreme Court building were shut down.

Mr Stevens's widow, Maureen, has sued the US Government for $50 million for the death of her husband, claiming that it failed to provide adequate security at the Fort Detrick facility.

“She wants to see a perpetrator brought to justice,” her lawyer, Richard Schuler, said. “Obviously, this gentleman Ivins has committed suicide. I don't think it was very effective to warn him that he was about to be charged and allow him to commit suicide like this. That seems pretty shoddy to me.”

Dr Ivins had recently begun treatment for depression, according to a report in the Los Angeles Times.

The investigators focused on him last year after he gave conflicting statements as to why he failed to report anthrax contaminations in areas where he worked, in late 2001 and early 2002, for five months.

FBI agents and officials from the Postal Inspection Service have conducted more than 9,000 interviews on four continents and served thousands of subpoenas.

They travelled to Afghanistan twice to chase up leads that proved to be false.

Investigators soon discovered that the anthrax used was the Ames strain, which is most commonly used in American biodefence research. The envelopes were all posted in Princeton, New Jersey, about 200 miles from the home of Dr Ivins in Frederick, Maryland.

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