With that, investigators linked the specific type of anthrax back to Ivins' biological weapons lab at Ft. Detrick in Frederick, Md., where he oversaw its use and handling for research.
"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 close to the investigation. "They were cultures he was personally responsible for."
The scientist spoke on condition of anonymity because he is not authorized to speak to reporters.
The scientific discovery gave the FBI its first solid break in one of the nation's most high-profile unsolved crimes after years of pointing the finger at the wrong suspect. Combined with other evidence, the Justice Department is expected to close the case this week, concluding Ivins was the mastermind and sole criminal behind the attacks that killed five and sickened 17 others in the weeks following 9/11.
Ivins killed himself last week as prosecutors prepared to indict him on murder charges.
Dozens of other researchers in Ivins' lab also had access to the type of Ames strain used in the attacks, the scientist said, meaning the DNA alone is not enough to prove his guilt.
Investigators have said they used other evidence to build the case against Ivins, including looking at who had access to the poison or the labs at the specific time it was mailed. Those details are expected to be spelled out in sealed court documents that are expected to be released this week if the Justice Department ends the investigation, possibly as early as Monday or Tuesday.
A senior law enforcement official said Sunday that victims' families were waiting to be briefed at FBI headquarters in Washington as soon as prosecutors agree to end the investigation.
Although the Army lab where Ivins worked had long been on the FBI's radar, scientists were unable to pinpoint the specific strain used in the attacks until about a year ago.
The FBI recruited top genome researchers from across the country and encouraged them to do groundbreaking work to identify and isolate the type of anthrax in the attacks. At least $10 million was spent on the research in what the scientist called the FBI's most expensive and scientifically compelling case to date.
The new genome technology that tracked down Ivins was either not available or too expensive to use often until about three years ago. It also looked at the DNA of the anthrax still in the envelopes that began showing up at congressional offices, newsrooms and post offices soon after Sept. 11, 2001.
The science is known as DNA fingerprinting. Although any two samples of anthrax bacteria will probably share roughly the same DNA structure, there are tiny differences from sample to sample. Scientists used those "fingerprints" to identify the source of the anthrax that killed five people.
In the years since scientists mapped the human genome, computer speeds have increased dramatically, making this process easier and less expensive. DNA fingerprint analysis that would have taken years not too long ago can now be done in days.
The government scientist said the FBI knew the DNA evidence linked Ivins to the attacks for at least a year. However, prosecutors worried that because the genome technology was so new, it might be questioned and eventually thrown out if the case against Ivins ever went to trial. Researchers tested it for many more months to make sure its conclusions were reliable.
Even so, its use in the anthrax case will probably spark scientific debate on how strongly it can be used to help solve crimes, the scientist said.
He predicted few would be able to argue with its conclusions -- namely, identifying the type of Ames strain used. Still, the scientist said, some researchers will probably note the DNA does not alone give the government a smoking gun or other surefire case-closer.