A month after the F.B.I. declared that an Army scientist was the anthrax killer, leading members of Congress are demanding more information about the seven-year investigation, saying they do not think the bureau has proved its case.

A month after the F.B.I. declared that an Army scientist was the anthrax killer, leading members of Congress are demanding more information about the seven-year investigation, saying they do not think the bureau has proved its case. 5

A month after the F.B.I. declared that an Army scientist was the anthrax killer, leading members of Congress are demanding more information about the seven-year investigation, saying they do not think the bureau has proved its case.

In a letter sent Friday to Robert S. Mueller III, the director of the Federal Bureau of Investigation, Democratic leaders of the House Judiciary Committee said that “important and lingering questions remain that are crucial for you to address, especially since there will never be a trial to examine the facts of the case.”

The scientist, Bruce E. Ivins, committed suicide in July, and Mr. Mueller is likely to face demands for additional answers about the anthrax case when he appears before the House and Senate Judiciary Committees on Sept. 16 and 17.

“My conclusion at this point is that it’s very much an open matter,” Senator Arlen Specter of Pennsylvania, the top Republican on the Senate committee, said of the strength of the case against Dr. Ivins, a microbiologist at the Army’s biodefense laboratory who worked on anthrax vaccines. “There are some very serious questions that have yet to be answered and need to be made public.”

Bureau officials say they are certain they have solved the nation’s first major bioterrorism attack, in which anthrax-laced letters killed five people, after a long and troubled investigation that by several measures was the most complex in the bureau’s history.

But in interviews last week, two dozen bioterrorism experts, veteran investigators and members of Congress expressed doubts about the bureau’s conclusions. Some called for an independent review of the case to reassure the public and assess policies on the handling of dangerous pathogens like anthrax.

Meanwhile, new details of the investigation, revealed in recent interviews, raised questions about when the bureau focused on Dr. Ivins as the likely perpetrator and how solid its evidence was.

In April 2007, after the mailed anthrax was genetically linked to Dr. Ivins’s laboratory and after he was questioned about late-night work in the laboratory before the letters were mailed, prosecutors sent Dr. Ivins a formal letter saying he was “not a target” of the investigation. And only a week before Dr. Ivins died did agents first take a mouth swab to collect a DNA sample, officials said.

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