WASHINGTON – Another postal facility was closed Tuesday as concern spread over the detection of anthrax in two military mailrooms. Hundreds of workers were offered antibiotics as a precaution, though no unusual health problems were reported.

Officials said the mail in question had been irradiated, so any anthrax in them was inert when they triggered alarms at the Pentagon mail facility and another nearby facility in the Virginia suburbs of Washington that handles military mail.

Environmental testing was being conducted on the two military mail facilities and on a third postal facility in the District of Columbia, which was closed Tuesday because it may have handled the mail that went to the two military mailrooms.

Antibiotics were offered to some 200 workers at the D.C. facility and to workers at the military mailrooms. Hospitals were told to be on the lookout for symptoms like respiratory problems, rashes or flu-like symptoms that could signal exposure to anthrax, which can be used as a biological weapon.

“This is a prudent course of action. I don’t think there’s cause for alarm or panic or undue worry,” said Dr. Gregg Pane, director of the city’s Department of Health. “We’ve also mobilized our strategic national stockpile so we have enough antibiotics available should the need arise.”

At the White House, spokesman Scott McClellan said President Bush (news – web sites) was told of the developments. “The initial testing came back positive. There was some additional testing that was done and it was inconclusive,” McClellan said. “We’re still waiting on more definitive results.”

Also on Tuesday, a hazardous materials team was called to a building occupied by the Internal Revenue Service (news – web sites) after a report of a powdery substance found in a letter. IRS officials said in a statement later that “initial tests were negative for chemical or biological substances.”

At the Pentagon, officials on Tuesday corrected information about how anthrax was discovered.

Spokesman Glenn Flood said that a filter from a scanning machine at the Pentagon’s mail handling facility, not a specific piece of mail, tested positive for anthrax. That filter was removed for testing on Thursday and the positive result came back Monday, prompting the closure of the facility.

Officials were tracking down some 8,000 pieces of mail that passed through the Pentagon’s facility for distribution, for further testing. Pentagon officials still didn’t know Tuesday how anthrax was detected at the nearby satellite facility in Fairfax County, Va.

The Pentagon’s mail delivery site, which is separate from the main Pentagon building, was evacuated and shut down Monday remained closed, along with the Fairfax County site.

Some 263 people were tested for anthrax exposure at the Pentagon site, officials said.

Follow-up tests were being conducted at the U.S. Army Research Institute for Infectious Diseases at Fort Detrick, Md., officials said. They would take two to three days to complete.

Anthrax can be spread through the air or by skin contact. Officials noted that sometimes anthrax sensors can give false-positive results.

In October 2001, someone sent anthrax in letters through the mail to media and government offices in Washington, Florida and elsewhere, raising fears of bioterrorism. Five people were killed and 17 more sickened. Those cases have never been solved.