Dr Ivins worked on anthrax vaccines Photo: Frederick News Post
The death of a US scientist suspected of anthrax attacks in 2001 was a result of "relentless accusation and innuendo", his lawyer has said.
A US medical examiner confirmed that Dr Bruce Ivins, 62, killed himself after being told he was about to be charged.
His lawyer said his client's innocence would have been proved in court.
Officials declined to give details of the probe but sources said prosecutors were to seek the death penalty over the attacks that claimed five lives.
The anthrax, which was posted to media offices and politicians shortly after the 11 September attacks in 2001, made another 17 people sick.
The sole deadly biological attack on US soil severely rattled a public already nervous of further terrorist action.
Dr Ivins worked for the US Army Medical Research Institute of Infectious Diseases (USAMRIID) in Fort Detrick, Maryland, and was actively involved in the investigation to find the perpetrators of the anthrax attacks.
ANTHRAX PANIC, 2001
First anthrax-laced letter is mailed on 18 Sept, 2001
Florida sees first of five deaths, three weeks later
The dead are two postal workers in Washington, a New York hospital worker, a Florida photo editor and an elderly woman in Connecticut
Panicked Americans try to stock up on antibiotic Cipro
Postal depots shut for de-contamination; mail is irradiated
Senate offices shut for weeks
Hoaxes become an almost daily occurrence
Plans to deal with a biological weapons attack updated
But the massive investigation that followed centred on one of Dr Ivins's colleagues, Dr Steven Hatfill, who later sued the justice department and won a $5.82m (£2.94m) settlement this June.
Last week, FBI director Robert Mueller told CNN "great progress" had been made in the investigation and he was confident it would be resolved.
The Los Angeles Times reported on Friday that Dr Ivins had been told he would be prosecuted and took the overdose of painkillers before dying in hospital on Tuesday.
His lawyer, Paul Kemp, said his client had fully co-operated with the authorities, adding: "We are saddened by his death and disappointed that we will not have the opportunity to defend his good name and reputation in a court of law."
He said: "The relentless pressure of accusation and innuendo takes its toll in different ways on different people, as has already been seen in this investigation. In Dr Ivins's case, it led to his untimely death."
A colleague of Dr Ivins at Fort Detrick, Russell Byrne, told Associated Press news agency the FBI had "hounded" Dr Ivins.
Dr Byrne said Dr Ivins had had to seek treatment for depression.
The LA Times said Dr Ivins was being forced to retire in September.
The justice department and FBI issued a brief statement on Friday saying "substantial progress has been made" but would give no further details.
Sources said a decision would be made in the next week whether to close the case - indicating that investigators believed Dr Ivins acted alone.
The Associated Press news agency said the authorities were investigating whether Dr Ivins had released the anthrax to test the vaccine he was developing.
Former CBS anchor Dan Rather recalls the day anthrax traces were found in his office
AP said Dr Ivins had complained that anthrax testing on animals was insufficient.
It also said Dr Ivins had been ordered to stay away from a social worker who was counselling him.
The worker, Jean Duley, said in filed court documents that Dr Ivins had "a history dating to his graduate days of homicidal threats, plans and actions towards therapists".
Ms Duley added that the doctor's psychiatrist had said Dr Ivins was homicidal and sociopathic.
Dr Ivins's brother, Tom Ivins, who had not spoken to him since 1985, said the social worker's comments "make sense".
"He considered himself like a god," Tom Ivins said.
In 2003, Dr Ivins, a married father of two, was awarded the highest honour for defence department civilian staff for his anthrax vaccine work.
In a statement on Friday, USAMRIID said it "mourned the loss of Dr Bruce Ivins, who served the institute for more than 35 years as a civilian microbiologist... we will miss him very much".
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