After four years of painstaking scientific research, the F.B.I. by 2005 had traced the anthrax in the poisoned letters of 2001 to a single flask of the bacteria at the Army biodefense laboratory at Fort Detrick, Md., according to government scientists and bureau officials.

But at least 10 scientists had regular access to the laboratory and its anthrax stock — and possibly quite a few more, counting visitors from other institutions, and workers at laboratories in Ohio and New Mexico that had received anthrax samples from the flask at the Army laboratory.