FBI agents have returned to an Army laboratory to search for clues in connection with the 2001 anthrax attacks that killed five people in the United States.

A two-and-a-half-year investigation in which the FBI has conducted more than 5,000 interviews, issued 4,000 subpoenas, and ordered several thousand hours worth of laboratory work keeps returning to Frederick, Maryland.

Frederick is home to the Army's Medical Research Institute of Infectious Diseases, located at Fort Detrick. It was there that the only person publicly named by U.S. officials in connection with the anthrax investigation, biological weapons expert Steven Hatfill, once worked.

An FBI spokesperson confirms to VOA that agents have again searched the laboratories at Fort Detrick for clues. The spokesperson declined to comment on what, if anything, might have been uncovered. An Army spokesman says laboratories at Ft. Detrick have been closed since late last week, coinciding with the FBI's probe.

Nearly one year after the anthrax attacks, Attorney General John Ashcroft named Steven Hatfill as a "person of interest" in the anthrax investigation. Agents twice searched his home. No charges have been filed.

Mr. Hatfill has steadfastly maintained his innocence. He has filed lawsuits against both the Department of Justice and The New York Times newspaper, accusing them of defamation.

An attorney for Mr. Hatfill declined to speak with VOA.

The 2001 anthrax incidents came on the heels of the September 11 terrorist attacks on the United States. For months, Americans were alerted to the possibility of suspicious "white powder" in envelopes and other containers sent through the U.S. postal system. The attacks sickened 22 people and claimed the lives of two Washington-area postal workers, a Florida photojournalist, a New York health care worker, and an elderly Connecticut woman.

Anthrax spores can either be inhaled or absorbed through the skin. The antibiotic known as Cipro has had mixed results in combating the bacteria.