US authorities have expressed confidence that a US bioweapons expert who committed suicide was the lone culprit behind the 2001 anthrax attacks that terrorised the United States.

Bruce Ivins (62) killed himself with a prescription drug overdose last week as prosecutors were preparing to charge him in the attacks that left five people dead and sickened 17 others, in a case that brought fears of bio-terrorism on the heels of the 11 September attacks.

After a seven-year-long investigation during which authorities wrongly named another scientist as a "person of interest" in the case, officials said they were wrapping up the probe and would declare the case closed.

Case closed

"Based upon the totality of the evidence we had gathered against him, we are confident that Dr. Ivins was the only person responsible for these attacks," US attorney Jeffrey Taylor told a news conference.

"We are now beginning the process of concluding this investigation," he said. "We will formally close the case."

Officials said they began focusing on Ivins as a suspect last year after new forensic science allowed them to trace the anthrax back to the scientist.

Investigators concluded that the anthrax that was mailed to prominent journalists and politicians in 2001 could have only come from a single flask of parent spores that only Ivins maintained and had created.

In a strange twist, Ivins, who worked for 18 years at the US biodefense research laboratories at Fort Detrick, Maryland, had been working on a vaccine against the disease that same year, Taylor said.

Wrestling with demons

But authorities painted a picture of a scientist wrestling with demons.

"Ivins had a history of mental health problems and was facing a difficult time professionally in the summer and fall of 2001 because an anthrax vaccine he was working on was failing," Taylor said.

In one email to a co-worker, Ivins stated that he had "incredible paranoid delusional thoughts at times" and feared he might not be able to control his behavior, Taylor said.

Recently, the federal official said, Ivins had made a threat in his group therapy session to kill people who had "wronged him" after he learned he might be indicted in the case.

The Federal Bureau of Investigation released on Wednesday documents from their massive investigation code-named Amerithrax, including dozens of search warrants, police reports and anonymous letters.

Officials said they were compelled to disclose the evidence even though no one was charged due to the high public interest in the case.

But Ivins' attorneys insisted that their client was innocent and that the US Department of Justice had no case.

"The government's press conference was an orchestrated dance of carefully worded statements, heaps of innuendo and a staggering lack of real evidence — all contorted to create the illusion of guilt by Dr. Ivins," his attorneys, Paul Kemp and Thomas DeGonia, said in a statement.

A loving father

"The government would have the American people believe that after seven years and more than $15-million of taxpayer money, they have found the individual responsible for the heinous attacks of the fall of 2001. Nothing could be farther from the truth," they said.

"In truth, Bruce Ivins was a devoted husband and father who worked for more than 30 years to defend his nation and its soldiers against the terrible effects of anthrax."

His lawyers said hundreds of soldiers, scientists and family members attended a funeral service for Ivins on Wednesday.

"No one who attended that service could believe that Dr. Ivins committed any crime," they said.

Since Ivins' suicide on 29 July, a mixed picture has emerged of the highly-decorated scientist, who had a moustache and hair neatly parted to the side.

Friends and colleagues have described him as a model citizen who played guitar in his church folk group, an avid gardener and an active volunteer with the Red Cross.

He was also said to be a loving father and husband to his wife of 33 years, Diane, and their 24-year-old twins.

AFP