Investigators say the fictitious return address left on some envelopes during the 2001 anthrax attacks may have been a reference to a Wisconsin school.

The return address used on some envelopes containing anthrax was "4th GRADE, GREENDALE SCHOOL" and its significance has long puzzled investigators.

A search warrant affidavit unsealed Wednesday says suspect Bruce Ivins may have been referring to the Greendale Baptist Academy in Greendale, Wisconsin, just outside of Milwaukee.

The affidavit says Ivins was a member of the American Family Association, a conservative Christian ministry that got involved in a 1999 lawsuit on behalf of the parents of a fourth grade student at the school.

Ivins committed suicide last week as prosecutors were preparing to charge him in the attacks.