Beginning in June, the government will administer the tests to workers at Fort Detrick, Md., about 40 miles northwest of Washington, and Dugway Proving Ground, Utah, about 85 miles southwest of Salt Lake City.
The government will focus on workers who had expertise in preparing anthrax for use as a weapon and those who may have had access to it, said the official, who spoke on condition of anonymity.
ABC News, which first reported the plans for testing, said some former employees of both facilities may be given polygraph tests as well.
The law enforcement official said the plan to test employees does not mean the government already has a suspect.
The investigation into who sent several anthrax-laced letters last year has produced few leads and investigators acknowledge the trail is growing cold. The government has begun a strategy of focusing on possible sources of anthrax rather than identifying suspects from the few clues gained from the letters.
Officials at Fort Detrick and Dugway did not immediately return telephone calls seeking comment Monday.
Army scientists in Utah have been developing a powdered form of anthrax for use in testing biological defense systems, military officials have said.
The Army said in a recent statement that small quantities of anthrax have routinely been produced at Dugway, and then shipped to the Army's biodefense center at Fort Detrick.
Fort Detrick, which also is home to the U.S. Army Medical Research Institute of Infectious Diseases, has anthrax samples from other sources as well.
Investigators began interviewing employees at Fort Detrick after anthrax-laced letters were sent to members of Congress in Washington and to television network offices in New York last year. Along the way, anthrax spores leaking from the letters contaminated post office buildings in Washington and New Jersey.
Two Washington postal workers died of inhaled anthrax, as did two women thought to have been infected from the mail. At least 13 people developed either skin or respiratory anthrax, but recovered.
The strain of anthrax found in letters mailed to Sens. Tom Daschle and Patrick Leahy is called Ames, after the city in Iowa where researchers first isolated it. Scientists at Detrick obtained a sample from the Agriculture Department in the early 1980s for vaccine testing and gave samples to at least five other labs.
Since the attacks, security at Fort Detrick has come under fire.
One former researcher at the infectious-disase center there said recently that nothing would have prevented workers from removing deadly germs from the labs.
"As far as carrying anything out, microorganisms are small," said Luann Battersby, a biologist who left voluntarily in 1998 after eight years. "The problem would be getting in, not getting out."
Another scientist, Richard Crosland, said supervisors did not often check whether researchers were keeping track of lab materials as required. When they did, some researchers just submitted photocopies of old reports, said Crosland, who was laid off in 1997.
Fort Detrick spokesman Charles Dasey declined to comment on the allegations of lax security.