A microbiologist who supervised the work of accused anthrax killer Bruce E. Ivins explained to a National Academy of Sciences panel Thursday why the arithmetic of growing anthrax didn't add up to Ivins' mailing deadly spores in fall 2001.

"Impossible," said Dr. Henry S. Heine of a scenario in which Ivins, another civilian microbiologist working for the Army, allegedly prepared the anthrax spores at an Army lab at Fort Detrick. Heine told the 16-member panel that Ivins would have had to grow as many as 10 trillion spores, an astronomical amount that couldn't have gone unnoticed by his colleagues.