FBI agents combed laboratory suites at Fort Detrick (search) — home to the Army's biological warfare defense program — on Tuesday, and a source said they were again looking for evidence in the 2001 anthrax attacks.

The labs at the U.S. Army Medical Research Institute of Infectious Diseases (search) have been closed since Friday, Fort Detrick spokesman Charles Dasey said.

A law-enforcement source, speaking on condition of anonymity, told The Associated Press the activity is related to the anthrax mailings that killed five people and sickened 17 in autumn 2001.

FBI agents have frequently visited Fort Detrick since the unsolved attacks, amid speculation that the deadly spores or the person who sent them may be connected to Fort Detrick.

Dasey said he didn't know which labs were involved, what sort of research had been conducted there or how long they would be closed.

Debra Weierman, spokeswoman for the FBI's Washington field office, said the lab probe was part of "an ongoing criminal investigation." She said could not discuss details of the activity.

Much of the speculation about a Fort Detrick connection has centered on Dr. Stephen J. Hatfill (search), a former government scientist and bioweapons expert who once worked at the infectious disease institute at Fort Detrick. The FBI has labeled Hatfill a "person of interest" in the case.

Hatfill has denied any role in the attacks. He has filed a lawsuit in U.S. District Court in Washington contending the government invaded his privacy and ruined his reputation by leaking information to the media implicating him in the attacks. His lawsuit seeks to clear his name and recover unspecified monetary damages.

His lawyer, Victor M. Glasberg, had no comment Tuesday.