Ten years ago, letters filled with powdered anthrax began arriving in mailboxes along the East Coast, killing five people in what became one of the worst biological attacks in U.S. history.

Today, two University of Wyoming professors say the man the FBI suspects of carrying out the attacks -- their former U.S. Army medical research colleague -- is innocent.

Assistant veterinary medicine professor Jeffrey J. Adamovicz will appear tonight on the PBS show "Frontline" to make the case that Bruce Ivins had neither the time nor the resources to mail anthrax-laced letters to several media outlets and two U.S. senators in the days following the Sept. 11, 2001 terrorist attacks.

Five people died and 17 others were sickened, sparking a nationwide fear about powder-filled letters that continues to this day.

Today, it's still unclear who sent the letters. Authorities initially suspected bioweapons expert Steven Hatfill, but focus soon turned to Ivins, who was developing a new anthrax vaccine at the U.S. Army Medical Research Institute of Infectious Disease in Frederick, Md.

The case against Ivins closed in July 2008, when he overdosed on Tylenol in an apparent suicide soon after learning that criminal charges were likely to be filed against him.

However, the controversy still continues, as the family of one of the anthrax victims has filed a civil lawsuit against the U.S. government alleging negligence in handling anthrax at the Army lab.

Government investigators compiled a list of circumstantial evidence against Ivins. He worked long hours and late nights alone, and he was one of the few people who had access to deadly strains of anthrax, a bacterial disease that's very rarely contracted in North America. His vaccine program wasn't going well, according to U.S. Attorney Jeffrey A. Taylor, and an anthrax attack could reinvigorate enthusiasm for his work.

But both Adamovicz, who was Ivins' supervisor, and UW assistant veterinary sciences professor Gerry Andrews, who worked with Ivins for years, said the FBI's case against their former colleague was very weak.

For one thing, they said, Ivins would have needed several months to prepare the amount of anthrax used in the attacks -- much longer than the couple of weeks alleged by the government.

Ivins also worked with wet spores, Adamovicz said, not the dry spores used in the attack. If Ivins dried his spores, he said, they wouldn't have the same composition as those found in the letters.

Ivins used a similar anthrax strain to the type found in the letters, Andrews said, but that same strain was also farmed out to a number of other labs.

"This is not rocket science," Andrews said. "I mean, the evidence that the FBI is proposing in support of their conclusion that Bruce Ivins did this is incredibly weak. And there's no real sound, scientific basis for this."

Also, having worked with him for years, both Adamovicz and Andrews said Ivins gave no indication of being someone who would want to send lethal bacteria to kill innocent people.

"He was a little quirky, a little bit socially awkward, but he was a very nice guy," Adamovicz. "He was a person who valued other people -- he wasn't a loner at all. ... This sort of thing is completely out of keeping with his personality."

Adamovicz said he has no guesses as to who was responsible for mailing the letters. That's difficult to tell, he said, as much of the investigation remains classified.

But his theory is that someone in one of the labs may have palmed a couple of glass vials and snuck them out.

"If you have an insider threat, it's going to be near to impossible to catch somebody trying to steal a ... strain or a small amount of biologics," Adamomicz said.

Adamomicz said the FBI didn't even ask for all the labs that had received samples of the anthrax strain -- possibly, he theorized, because the government was manufacturing anthrax secretly in violation of the Biological Weapons Convention.

"It's just my local conspiracy theory, but I can't imagine why any investigator worth their salt wouldn't pursue that as a central line of questioning," he said.

In the years following the anthrax attacks, Andrews, then Adamovicz, left the Army lab to take faculty positions at the University of Wyoming.

The two are currently helping to develop a brucellosis vaccine for elk and bison, which may transfer the disease to livestock, causing aborted or stillborn calves and weakened adult animals.

But even 10 years and thousands of miles removed from Bruce Ivins, Adamovicz said the mystery of the anthrax attacks is still a major part of his life.

"It's almost like the White Whale in a way," Adamovicz said, referring to Captain Ahab's continually out-of-reach nemesis in Moby Dick. "It is something that I feel morally obligated to continue to pursue."

Contact capital bureau reporter Jeremy Pelzer at 307-632-1244 or jeremy.pelzer@trib.com.