"We are now beginning the process of concluding this investigation [and will] formally close the case."

Investigators from the FBI and the US Postal Inspection Service concluded the anthrax, mailed to prominent journalists and politicians in 2001 shortly after the September 11 attacks on the World Trade Centre and Pentagon, came from a single flask of parent spores that only Ivins maintained and had created.

The authorities had earlier revealed that Ivins, who worked for 18 years at the US biodefence research laboratories at Fort Detrick in the US state of Maryland, had been working on a vaccine against the disease.

Mental health issues

Ivins reportedly could not give investigators an explanation for his late-night laboratory work hours around the time of both anthrax mailings.

He even submitted false samples of anthrax from his lab to the FBI to mislead investigators, Thomas Dellafera, an investigator for the postal inspection service, said.

He had also told a colleague that he was suffering serious mental health issues in the months preceding the attacks and had "incredible paranoid, delusional thoughts at times", which led to him to fear he could not control his behaviour, Dellafera said.

However, Ivins's lawyers have rejected the allegations, saying 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 lawyers, Paul Kemp and Thomas DeGonia, said.

Critics have questioned the evidence against Ivins, particularly after investigators mistakenly named Steven Hatfill, another scientist at Fort Detrick, as a "person of interest" over the attacks.

Hatfill was never charged and in June this year the US government agreed to pay him $5.85m to settle a lawsuit he had brought.

In addition to the five deaths, 17 people became ill and the US postal service was thrown into chaos following the attacks.