in the letters. The FBI noted that one or more derivative growth steps would have been required to produce the anthrax in the attack letters. Furthermore, the contents of the New York and Washington letters had different physical properties.

The FBI's scientific data did provide leads as to the origin of anthrax spores in the letters, the committee found; however, the data did not rule out other possible sources. The committee recommended that realistic expectations and limitations regarding the use of forensic science need to be clearly communicated to the public.

During the next seven years, the task force expended more than 600,000 investigator work hours, interviewing thousands of witnesses on six continents and evaluating a mountain of evidence. In the end, however, the agency's case against Ivins was largelycircumstantial.

Ivins killed himself in July 2008, just as the Justice Department was about to formally indict him.

Before his death, Ivins was a microbiologist and researcher at the U.S. Army Medical Research Institute of Infectious Diseases, the federal facility that develops countermeasures for potential biological weapons agents.

The evidence gathered against him confirms that there were anthrax spores in Ivins' laboratory, and Ivins made a series of unusual, after-hours visits to his lab just before the attacks occurred. But no one reported seeing him mail the letters; no one said they saw him prepare the anthrax spores found in the letters; and no one could provide a clear motive for why he would carry out such an attack.

The FBI and Justice Department have maintained that a scientific analysis of the