Traces of the deadly bacteria anthrax were found in two Pentagon mail facilities Monday. The facilities were immediately closed and as many as 300 workers were sent for testing to determine if they had been exposed.

Subsequent tests Monday came back negative for anthrax but further tests are being conducted at the U.S. Army Research Institute for Infectious Diseases at Fort Detrick, Maryland. Results of these follow-up tests are expected back in 2-3 days.

The first possible traces of anthrax were detected by a sensor on Thursday of last week at the Pentagon Remote Delivery Facility. The second facility is actually located a few miles away from the Pentagon.

Officials at the Pentagon said that all incoming mail is X-rayed and irradiated to kill any bacteria such as anthrax that may be inside.

Anthrax attacks were made just after the September 11 attacks at the Washington, DC, offices of Senators Tom Daschle and Patrick Leahy as well at television network news offices in New York. Five people died then from exposure. The people that sent the deadly bacteria were never positively identified.