WASHINGTON — A former Army microbiologist who worked for years with Bruce E. Ivins, whom the F.B.I. has blamed for the anthrax letter attacks that killed five people in 2001, told a National Academy of Sciences panel on Thursday that he believed it was impossible that the deadly spores had been produced undetected in Dr. Ivins’s laboratory, as the F.B.I. asserts.

Asked by reporters after his testimony whether he believed that there was any chance that Dr. Ivins, who committed suicide in 2008, had carried out the attacks, the microbiologist, Henry S. Heine, replied, “Absolutely not.” At the Army’s biodefense laboratory in Maryland, where Dr. Ivins and Dr. Heine worked, he said, “among the senior scientists, no one believes it.”

Dr. Heine told the 16-member panel, which is reviewing the F.B.I.’s scientific work on the investigation, that producing the quantity of spores in the letters would have taken at least a year of intensive work using the equipment at the army lab. Such an effort would not have escaped colleagues’ notice, he added later, and lab technicians who worked closely with Dr. Ivins have told him they saw no such work.

Image A hazardous-materials team investigated a post office in West Trenton, N.J., in October 2001 for clues in the anthrax attacks.

He told the panel that biological containment measures where Dr. Ivins worked were inadequate to prevent the spores from floating out of the laboratory into animal cages and offices. “You’d have had dead animals or dead people,” he said.