They meet with local residents, listen to eye-witness reports, study documentation, conduct forensic experiments on physical evidence, and conduct on-location archaeological studies.

That sample, the size of a sugar packet, has led the F.B.I. to commission a series of sophisticated forensic experiments by outside laboratories to try to determine when, where and how the anthrax was produced.

Police officials said their forensic experiments had concluded that the explosive used in the two devices was TNT, the material most frequently used by Islamic militants operating from the West Bank and Gaza.