The labs at the U.S. Army Medical Research Institute of Infectious Diseases 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, 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.
The FBI had Hatfill under 24-hour surveillance for many months following the attacks. In one incident, agents in a vehicle trailing Hatfill ran over his foot on a Washington street. Hatfill was not seriously hurt, and the surveillance has been curtailed.
The powder-filled envelopes sent fear through a nation already shaken by the Sept. 11 attacks and left people worried about their mail.
After American Media Inc. photo editor Robert Stevens died from inhaling anthrax at a building in Florida, four more deaths followed from letters containing anthrax that were sent to media outlets and the Capitol Hill offices of Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt.
Postal facilities closed, as did office buildings on Capitol Hill, where hundreds of lawmakers, staff members and others were tested and given an antibiotic.
At the Brentwood facility, two postal workers died from inhalation anthrax.
Anthrax is caused by bacteria that can be passed from livestock to humans. The disease can affect the skin, the lungs or the digestive system. When treated appropriately, less than one percent of people with the cutaneous, or skin form, of anthrax die. But more than half of those who contract inhalation or gastrointestinal anthrax perish.