A grand jury investigation was said to be planning an indictment seeking the death penalty over the attacks, which left five people dead and caused widespread panic in the US shortly after the September 11, 2001, attacks.

A spokeswoman for the FBI did not comment on the reports.

However Ivins' lawyer told AP his client had been innocent and had been co-operating with investigators for more than a year.

"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,'' said Paul F Kemp.

'Closing in'

The bio-defence laboratory at Fort Detrick and its scientists had been at the centre of the FBI's investigation for several years, the Associated Press news agency says.

"Single individuals are capable of doing a tremendous amount of damage economically and also in terms of lives lost."

Charles Blair, counter-terrorism analyst

In recent months authorities had been "closing in" on Ivins, a skilled microbiologist who allegedly helped the FBI to analyse evidence recovered from one of the anthrax-tainted envelopes used in the attacks, the LA Times reported.

Ivins had spent more than 10 years working to develop an anthrax vaccine that was effective even in cases where different strains of anthrax were mixed, which made vaccines ineffective, and authorities were investigating whether Ivins released the anthrax as a way to test his vaccine, officials told AP.

W Russell Byrne, a former colleague of Ivins at Fort Detrick, said Ivins was "hounded" by FBI agents who raided his home twice.

He had been also removed from his job by local police recently because of fears he had become a danger to himself or others, AP reported.

However Byrne said he did not think Ivins was behind the attacks.

Lawsuit

The letters, laced with the deadly anthrax spores, were sent via the US postal system to several US politicians, including senators Patrick Leahy and Tom Daschle, television networks in New York and tabloid newspaper offices in the southern state of Florida.

Five people died and 17 others became ill, the US postal service was also crippled, several government buildings were temporarily shut down and many feared the attack was by groups such as al-Qaeda.

In late June this year the US government settled a lawsuit brought by a former colleague of Ivins, Steven Hatfill, who sued the government after it named him as a "person of interest" in the anthrax investigation.

The case against Hatfill was dismissed and the US government paid him $5.82 million.

Charles Blair, a counter-terrorism analyst whose organisation is funded by the US department of Homeland Security, told Al Jazeera that the case, while not proven, reveal a lot about the damage that could be done by individuals.

"What this reveals is the real threat of the 'lone wolf' that we have now"...and it "tells us a lot about the changing trajectory of asymmetrical warfare."

"Single individuals are capable of doing a tremendous amount of damage economically and also in terms of lives lost."