He also said that so many people were going in and out of the labs at odd hours to check on experiments that it's unlikely Ivins would have gone undetected if he had been working on something illicit, even at night.

The FBI said Ivins used a machine called a lyophilizer in his lab to make the anthrax. But such a machine is also used in creating anthrax vaccines.

Why did two of the anthrax letters include a harmless bacterial contaminant, but not the others? The FBI has not shown where that bacterium originated or if agents tried to trace it to Ivins. Nor has there been an explanation of why the first set of letters contained the bacterium but not the second set.

Experts would also like to see more detail from the FBI on how exactly it was able to link the anthrax used in the mailings to the flask handled by Ivins.

"So much of the FBI's case is based on the fact they are 100 percent convinced it came out of that one container," said Randall Larsen, national security adviser for the Center for Biosecurity at the University of Pittsburgh Medical Center. "Even the FBI themselves admit this is a brand-new technology they came up with."

•What motive would Ivins have had? The FBI suggested he wanted heighten the need for an anthrax vaccine he was working on. The FBI also released Ivins' own e-mails that describe his mental state as depressed, delusional and paranoid.

But does that make him a killer?

"He's obviously a disturbed individual, but how that disturbed behavior exhibited itself does not seem at all relevant to engaging in a bioterrorist attack," said David Fidler, director of the Indiana University Center on American and Global Security.

And Byrne said he never saw bizarre behavior that went beyond what might be expected from an eccentric scientist.

"If he had mental health problems, he was taking care of them well," Byrne said. "Could he have been so smart that he completely fooled me?

"Yeah, it's possible, but I doubt it."