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Known as the “Amerithrax” investigation, Mr. Persichini called the FBI probe one of the largest and most complex ever conducted by law enforcement. He said the DNA testing allowed investigators to pinpoint the origins of the anthrax to Mr. Ivins’ lab.

The first concerns about contamination at Fort Detrick surfaced in December 2001, when “positive cultures” for the anthrax bacteria were found outside the biocontainment area of Building 1425, the report said.

According to the report, a microbiologist in the building, whose name was redacted, identified the cultures and decontaminated the area, although he did so “on his own initiative and without the direction of any supervisors.” It said the microbiologist also did not notify any supervisors of his actions or of the contamination.

While the microbiologist’s name is protected in the report’s narrative on how and why the Army’s investigation began, an accompanying index of the report’s exhibits identifies him as Mr. Ivins. His name also appears two other times in the index in connection with redacted sworn statements he made.

In the report, Mr. Ivins’ name appears to have been mistakenly listed on several occasions, suggesting he was a major player in the U.S. Army probe. In one reference, “Bruce” was redacted but “Ivins” was left in; a handwritten note on a totally redacted page says, “Bruce Ivins statement.” In another handwritten note on a two-page sworn statement in which the source is protected, “Ivins” is listed in the margin.

According to the report, the Fort Detrick contamination first became known to lab supervisors on April 16, 2002, and was confirmed two days later as the anthrax bacteria. The report said evidence gathered by Army investigators suggested that “a single source” was responsible for the contamination although it neither identified a suspect nor ruled out the possibility that others could have been involved.

The report also said that “multiple contaminates” identified in an extensive search of the facility “suggest that multiple episodes of contamination may have occurred.” The source of the decontamination was not discovered, the report said.

Mr. Ivins was working on finding vaccines for the anthrax bacteria. He and a colleague held two U.S. patents for anthrax vaccine technology, but law-enforcement officials have theorized that Mr. Ivins’ decaying mental health and his desire to show people the importance of his vaccines could have motivated him to carry out the worst bioterrorism attack in the nation’s history.

In October 2001, several letters contaminated with a virulent version of the anthrax virus were mailed to the offices of Democratic Sens. Tom Daschle of North Dakota and Patrick J. Leahy of Vermont, as well as to the offices of ABC News, CBS News, NBC News, the New York Post and the National Enquirer.

Mr. Ivins was among three Fort Detrick scientists to receive in March 2003 the Decoration for Exceptional Civilian Service - the highest award given to Defense Department civilian employees - for helping solve technical problems in the manufacture of anthrax vaccine.

Although the FBI had initially settled on another Fort Detrick scientist, Steven Hatfill, as the major focus of its anthrax investigation, the target changed in 2006 when FBI Director Robert S. Mueller III named veteran agent Vincent B. Lisi and Edward W. Montooth to lead the probe. The director told them to re-examine leads in the case and re-evaluate others who might have been overlooked as potential suspects.

Federal law-enforcement authorities said that was when Mr. Ivins emerged as a major suspect.

Mr. Hatfill later collected $5.8 million from the government as part of a settlement in a lawsuit he filed against the Justice Department and others.