WASHINGTON (CNN) -- FBI officials, while admitting a mistake, are offering more evidence to support their assertion that government scientist Bruce Ivins was responsible for the anthrax-laced mailings that killed five people in 2001.

In a two-hour briefing Monday at the agency's headquarters, senior FBI scientists -- backed by a panel of outside experts -- revealed they examined more than 1,000 anthrax samples and interviewed all of the approximately 100 people who had had access to the flask that reportedly held the deadly strain implicated in the killings.

In the briefing, intended to counter widespread questions about the investigation, the scientists fiercely defended the forensic process that led to their conclusion but acknowledged that they may never be able to satisfy everyone.

"We'll never put all the questions to rest," said Vahid Majidi, head of the FBI's Weapons of Mass Destruction Directorate. "There's always going to be a spore on a grassy knoll."

Majidi, FBI Laboratory Director David Hassell, six outside scientists, and academicians and several scientists from the FBI laboratory in Quantico, Virginia, joined the news conference.

The credibility of the FBI and the Justice Department took a hit after their initial identification of another scientist from the same lab, Steven Hatfill, as "a person of interest" in the probe in 2002. Hatfill was subsequently cleared and successfully sued the government.

The investigation then stretched on with no public evidence of progress until Ivins, facing an indictment, reportedly took his life last month.

The scientists said their investigation lasted from 2001 to 2008 because they were using new genetic analytic processes that were unfolding even as the investigation proceeded.

"It wasn't until late 2005 that the genetics caught up with the investigation," Hassell said.

"We learned as we went along," said Claire Fraser-Liggett, a University of Maryland scientist and adviser to the FBI investigation.

The learning curve involved at least one mistake. The FBI officials described having received an anthrax sample from Ivins in an early stage of the investigation but said they destroyed it because it failed to meet the requirements of a subpoena.

They acknowledged Monday that that was a mistake.

"We were not perfect," Majidi said. "Obviously, we would do things differently today."

After Ivins later submitted a second sample deemed acceptable, scientists discovered it did not contain some of the mutations that were in a strain investigators were focusing on.

They at first viewed the change as "deceptive" but said they now consider it as simply "questionable."

The scientists, who would not discuss Ivins' possible motives, focused solely on the scientific evidence, which they said increasingly pointed to two flasks in his corner of a government laboratory near Frederick, Maryland.

In Ivins' possession was a particular strain of anthrax known to scientists as RMR-1029. Investigators using new genetic analytical processes determined, primarily from examining the anthrax letters and envelopes, that eight samples contained four mutations, all of which led to the flask in Ivins' laboratory.

Ivins' lawyers maintain his innocence, saying that more than 100 people had access to the strain. The FBI says its investigation cleared all of them but Ivins.

The scientists said more than 60 experts from a wide range of institutions helped in the investigation and agreed with the scientific methods used.

More than a dozen outside scientists independently linked the anthrax samples to the type of anthrax found in Ivins' flask, they said.

The scientists said the anthrax in the letters could have been created in three to seven days by one person using the type of equipment located at the U.S. Army Medical Research Institute in Fort Detrick, Maryland, where Ivins worked.

Although authorities had previously said the anthrax in the attacks was "weaponized," coated with a special substance to make it disperse in the air, the scientists at the briefing said that was not the case.

The genetic analysis used in the investigation may yield payoffs in the future, said Fraser-Liggett. "This has application to homeland security -- our food, water and outbreaks," she said.

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