Naba Barkakati, an engineer who is the chief technologist for the Government Accountability Office and who also attended this week’s briefing, said of the F.B.I.’s forensics case against Dr. Ivins: “It’s very hard to get the sense of whether this was scientifically good or bad. We didn’t really get the question settled, other than taking their word for it.”

The bureau’s lab work has come under sharp criticism in recent years for problems over DNA analysis, bullet tracing and other important forensic technology. In 2004, the laboratory mismatched a fingerprint taken from the Madrid terror bombings to a lawyer in Portland, Ore., Brandon Mayfield, who was then arrested. He won a $2.8 million settlement.

With the main suspect in the anthrax killings now dead, F.B.I. officials say they realize they will again face tough scrutiny over the strength of their scientific evidence against Dr. Ivins. Indeed, conspiracy theories are already flourishing on many Web sites, with skeptical observers asking whether the Maryland scientist was set up to take the fall for the attacks or, worse yet, was a murder victim. The fact that the bureau pursued another scientist, Steven J. Hatfill, for years before agreeing to pay $4.6 million to settle a lawsuit he had filed and then later exonerating him has only fueled the skepticism.

“Do you believe Bruce Ivins was responsible for the anthrax attacks?” The Frederick News-Post, the hometown newspaper in Fort Detrick, asked its readers this week. (Of those who responded, 34 percent said no, compared with 26 percent who said yes.)

In its case against Dr. Ivins, the F.B.I. developed a compelling profile of an erratic, mentally troubled man who could be threatening and obsessive, as in his odd fascination with a sorority from his college days. But investigators were never able to place him at the New Jersey mailboxes where the anthrax letters were dropped, and the case against him relied at its heart on the scientific evidence linking the anthrax in Dr. Ivins’s laboratory to the spores used in the attacks.

It took the F.B.I. several years to develop the type of DNA testing that allowed them to trace the origins of the “attack strain,” as it was called, and they concluded that the anthrax that Dr. Ivins controlled was the only one of more than 1,000 samples they tested that matched it in all four of that strain’s genetic mutations.

Dwight Adams, a former director of the F.B.I. laboratory who was deeply involved in managing the anthrax genetic research until he left the bureau in 2006, said he was confident that the groundbreaking forensic effort would be validated by the broad scientific community.