Anthrax attacks caused widespread disruption across the US
A top US scientist suspected of anthrax attacks in 2001 has apparently killed himself just as he was about to be charged, a newspaper reported.
The Los Angeles Times said Bruce Ivins, 62, had taken an overdose of painkillers. It said he had recently been told of the impending prosecution.
There has been no official comment but unnamed sources said prosecutors were to indict and seek the death penalty.
Five people died when anthrax was posted to the media and politicians.
The incidents took place shortly after the 11 September attacks in 2001.
Security measures in the wake of the anthrax attacks temporarily closed a Senate building and increased the public's fear of their vulnerability to terrorism.
As well as the five deaths, 17 other people were made ill.
Dr Ivins worked for the US Army Medical Research Institute of Infectious Diseases (USAMRIID) in Fort Detrick, Maryland.
ANTHRAX PANIC, 2001
First anthrax-laced letter is mailed on 18 Sept, 2001
Florida sees first of five deaths, three weeks later
Panicked Americans try to stock up on antibiotic Cipro
Postal depots shut for de-contamination
Senate offices shut for weeks
Hoaxes become an almost daily occurrence
Plans to deal with a biological weapons attack updated
Mail irradiated to kill anthrax spores
As a microbiologist he helped the FBI investigate the anthrax-tainted envelopes.
The mail was sent to legislators in Washington and media offices in New York and Florida.
Those killed were two postal workers in Washington, a New York hospital worker, a Florida photo editor and an elderly woman in Connecticut.
Last week, FBI director Robert Mueller told CNN "great progress" had been made in the investigation and he was confident it would be resolved.
Associated Press news agency has quoted unnamed officials as saying an indictment was planned that would seek execution.
The agency said the authorities were investigating whether Dr Ivins had released the anthrax to test the vaccine he was developing.
Dr Ivins died in hospital on Tuesday. 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".
Dr Ivins's lawyer, Paul Kemp, said: "We assert his innocence in these killings, and would have established that at trial."
The FBI focused more on Dr Ivins after the leadership of the investigation was changed in 2006.
Another doctor, Steven Hatfill, won millions in a lawsuit
Another scientist at the Fort Detrick laboratory, Dr Steven Hatfill, who had been named a "person of interest" in the case in 2002, was exonerated of any involvement.
He sued the justice department, saying it had violated his privacy rights by speaking to reporters about the case.
In June, the US department paid Dr Hatfill $5.82m (Ł2.94m) to settle the lawsuit.
The LA Times said the investigation had been making new progress based on analysis of anthrax-tainted letters posted to senators Patrick Leahy and Thomas Daschle.
The paper said Dr Ivins would have been forced to retire in September.
A doctor who worked with Dr Ivins at Fort Detrick, Russell Byrne, told Associated Press news agency the FBI had "hounded" Mr Ivins.
Dr Byrne said Dr Ivins had had to seek treatment for depression.
In 2003, Dr Ivins was awarded the highest honour for defence department civilian staff for his anthrax vaccine work.