More than a thousand World Bank employees worked from home after an anthrax scare Tuesday, and a sister agency found evidence of spores in its mail room.

With no discernible progress on the investigation months after the anthrax letter attacks, the FBI prepared to give lie detector tests to current and former workers at a pair of Army medical labs where the microbes are on hand.

At Fort Detrick, Md., officials were reviewing a Defense Department proposal for stepping up security at the germ warfare defense lab, which has considerable anthrax stores. The goal: Secure biological agents as tightly as the nation secures chemical and nuclear material.

The directive, still in draft form, focuses on safe handling of the biological agents, screening personnel who have access to the material, securing the physical plant and tracking methods to account for supplies, said Caree Vander Linden, spokeswoman for U.S. Army Medical Research Institute of Infectious Diseases.

Former Fort Detrick employees have charged that lab security at the Army base has been lax. The FBI has made Fort Detrick, located outside Washington, a centerpiece of its investigation.

In Washington, officials at the World Bank directed 1,200 of its 8,500 employees to work from home for two days after a preliminary test of mail came back positive for anthrax. A second test showed no anthrax, and the bank is now awaiting results from a third, definitive test.

Preliminary field tests often inaccurately detect the presence of anthrax and are not considered significant unless confirmed.

Just in case, officials shut down the ventilation system connected to the room where the mail was, spokeswoman Caroline Anstey said Tuesday. Without air conditioning in offices where the 1,200 workers are located, it was too hot and they were sent home, Anstey said.

"We would prefer to overreact than underreact," she said.

Also Tuesday, the International Monetary Fund reported that it had received a positive reading from a batch of mail at its facility. Similarly, the IMF had one negative and one positive test and is awaiting results of a third screening.

In the investigation, FBI agents have already administered a round of polygraphs at Fort Detrick, focused on those who have handled evidence related to the anthrax probe, said Dr. John Ezzell, an anthrax expert there who is participating in the investigation.

The FBI maintains a presence to observe laboratory work related to the investigation, but agents who interviewed workers last fall and winter have been less active lately, Ezzell said.

The investigation has rattled workers, said Norman Covert, who is retired from Fort Detrick and stays in close touch with many of his former colleagues.

"They're a little bit intimidated by the attention they're getting by the FBI," he said.

The next round of polygraph tests, which could top 200, will be given to current and former employees at Fort Detrick and Dugway Proving Ground in Utah, a law enforcement official said Tuesday. The tests are voluntary, though some workers who are typically subject to the tests for security reasons could be compelled, he said.

Army scientists at the Utah facility have been developing a powdered form of anthrax for use in testing biological defense systems, military officials have said. Because the anthrax used in the attacks was also in powdered form, investigators have looked carefully at Dugway.

A parallel track in the investigation — attempting to use genetic fingerprinting to pinpoint the lab where the attack anthrax came from — still has not produced usable information, the official said.

Five people died in the anthrax attacks last fall, and at least 13 others contracted and recovered from either the skin or respiratory form of the disease.