Dr Ivins worked on anthrax vaccines (photo: Frederick News Post)
A new DNA identification process led US investigators to an army scientist said to be behind a lethal 2001 anthrax attack, a government scientist says.
The unnamed source told the Associated Press that Dr Bruce Ivins, 62, had been a prime suspect for more than a year for the deaths of five people.
He killed himself last week after being told he would face murder charges.
The anthrax was posted to politicians and media offices shortly after the 11 September 2001 attacks.
A lawyer for Dr Ivins said after his death that he had suffered "relentless accusation and innuendo", and that his innocence would have been proven in court.
The sole deadly biological attack on US soil, which also left 17 people ill, unsettled a public already nervous of further terror attacks.
DNA taken from the bodies of the five people killed helped lead investigators to Dr Ivins, who oversaw the highly specific type of toxin in an army biological weapons laboratory in Fort Detrick, Maryland, the government scientist said.
Using new genome technology, researchers looked at samples of cells from the victims to identify the kind of anthrax Ames strain that killed them, the scientist said.
They noticed very subtle differences in the DNA of the strain used in the attacks than in other types of Ames anthrax.
Spores taken from envelopes used to mail the anthrax, as well as from the sites where they were sent, also were scrutinised.
"It had to do with the very specific characteristics in the DNA of the letters and what was in Bruce's labs," said the government scientist, who is said by AP to be close to the investigation.
"They were cultures he was personally responsible for."
Dr Ivins had been actively involved in the investigation to find the perpetrators of the anthrax attacks.
Sources said earlier that a decision would be made in the next week whether to close the case - indicating that investigators believed Dr Ivins had acted alone.