Documents List Essential Clues

Investigators had recovered four envelopes used by the bioterrorist, and each was a pre-stamped, 6 3/4 -inch envelope that bore an image of an eagle in the upper right-hand corner. As it happened, the four envelopes also had a tiny printing defect.

While about 45 million such envelopes were made, only a small number had the printing defect, and all of them were sold at post offices in Virginia and Maryland, including the Frederick post office where Ivins rented a mailbox.

Eventually, Taylor said, investigators concluded that "the envelopes used in the mailings were very likely sold at a post office in the greater Frederick, Md. area."

Bureau investigators also connected the fictitious return address on the second round of anthrax letters -- the "Greendale School" of Franklin Park, N.J. -- to a charity well-known to Ivins. He had donated numerous times to a group called the American Family Association, which in 1999 had filed a lawsuit on behalf of parents at the Greendale Baptist Academy in Wisconsin in a dispute involving corporal punishment.

Ivins's unusual working habits around the time of the attacks provided another clue, documents show. After keeping relatively consistent working hours during the winter and spring of 2001, the scientist began spending far more time at his lab from mid-August to October of that year. The anthrax letters are believed to have been mailed in two batches, in September and October.

Time stamps from security logs showed Ivins making rare late-night visits to the B3 biosecurity chamber where the flask of RMR-1029 anthrax spores was kept. Ivins normally worked from 7:30 a.m. to 4:45 p.m., but records showed a string of weekend and after-hours visits in September and October of 2001. Often during these visits he would work until after midnight, when no other researchers would be present.

When asked about the strange schedule in 2005, Ivins could offer no explanation. He told investigators only that he worked late during those weeks "to escape" life at home.

The FBI was unable to find evidence of legitimate work Ivins performed during those visits.