Two buildings at a U.S. military base have been partially shuttered as part of a federal probe into the anthrax attacks (search) of 2001, FOX News has learned.

Investigators with the "Amerithrax" task force (search), which comprises FBI and postal service agents, have taken their three-years-long hunt to the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) at Fort Detrick, Md.

It has long been speculated that the anthrax in the tainted mailings that killed five people and sickened 17 shortly after the Sept. 11 attacks was somehow stolen from the Fort Detrick labs.

Dr. Steven Hatfill (search), described as a "person of interest" in the investigation, also worked at the USAMRIID facilities. He has denied any involvement in the lethal mail campaign, and is suing the U.S. government for publicly linking him to the attacks.

Even if Hatfill is completely ruled out as a suspect, the task force's interest in Fort Detrick leaves open the disturbing possibility that the culprit may well be an American — even a military officer.

Sources close to the investigation told FOX News that access to two USAMRIID (search) buildings that contain labs and are described as research "hot zones" has been limited.

"The U.S. Army Medical Research Institute of Infectious Diseases is temporarily closing some of its laboratory suites in support of an ongoing FBI criminal investigation," a statement issued from Fort Detrick said. "The temporary closing of laboratory suites is not associated with any incident that could pose a public or occupational health threat. Normal research operations will continue in the remainder of USAMRIID laboratories."

A federal judge is expected to decide soon whether to throw out parts of the lawsuit filed by Hatfill against Attorney General John Ashcroft (search) and others. Hatfill contends that his reputation was ruined when Ashcroft labeled him a "person of interest" in the anthrax attacks.

Earlier this month, the Justice Department revealed details of its probe into the attacks to U.S. District Judge Reggie B. Walton. A department spokesman declined to describe what details the FBI's lead investigator in the October 2001 anthrax attacks, Richard L. Lambert, gave to the judge, noting the document was classified. The information was not given to defense lawyers for Hatfill.

The FBI considers the anthrax investigation its most complex ever. About a month after the Sept. 11 attacks, letters laced with anthrax were mailed to government and news media offices, including to Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt., and to The New York Post and NBC anchor Tom Brokaw. The mailings rattled an already shaken populace, and postal and government buildings experienced closures. No one has yet been charged in the attacks.

Hatfill is a former government scientist and bioweapons expert who once worked at USAMRIID. The facility housed the strain of anthrax found in the envelopes sent to the victims, though Hatfill maintains he never worked with the bacterium, bacillus anthracis, that causes the infectious disease.

FBI Director Robert Mueller has said the Amerithrax investigation is focused on scientific tests to learn how the anthrax was made and who might have been capable of making it.

Walton previously decided to delay Hatfill's civil lawsuit until at least October. That decision came after Lambert told him a delay was "critical to the integrity and successful resolution" of the anthrax investigation. He warned that Hatfill probably will seek copies of sensitive internal FBI documents that would reveal names of people cooperating with investigators and other closely held details about evidence that has been collected.

Disclosing such information would provide "a voyeur's window" into the investigation, Lambert said, and give Hatfill or others ideas about destroying or hiding evidence, retaliating against witnesses or fleeing the country.

Hatfill's lawyers have described the government's assertions as a "parade of horribles" and said the information sought would not jeopardize the investigation.

FOX News' Catherine Herridge, Anna Stolley and The Associated Press contributed to this report.