WASHINGTON – The mayor of the nation's capital offered all workers at a local postal facility a three-day course of antibiotics Tuesday after it was determined that the post office had been the source of anthrax (search)-tainted mail sent to two military mail facilities in Virginia a day earlier.
The facility was closed while environmental crews determined whether the place was contaminated by the often-deadly bacteria. The city's chief medical officer reported no cases of the illness reported to local hospitals, but called distribution of the antibiotics the "proper first step."
"The postal service is taking all precautions by distributing prophylactic antibiotics and has asked us to help in the distribution today. We are offering prophylactic medicine in the form of antibiotics here at D.C. General (hospital) for all workers from the V Street postal service facility," D.C. Mayor Anthony Williams (search) said. About 200 employees would be eligible for the antibiotic Cipro.
Also on Tuesday, an Internal Revenue Service building was the site of another letter containing a powdery substance. IRS officials said in a statement later that "initial tests were negative for chemical or biological substances."
Last Thursday, sensors at two military mail facilities in the Washington area detected signs of anthrax on two pieces of mail, but the mail had already been irradiated (search), rendering any anthrax inert, defense officials told FOX News. Results returned on Monday showed a positive test for anthrax
Officials weren't sure if this was an attack. Additional tests and other sensors at the two facilities, one of them at the Pentagon and another at an office nearby, found no presence of the bacteria, which can be used as a biological weapon. The Pentagon reported no initial cases of illness.
President Bush was being regularly updated of the situation as testing continued on Tuesday, White House press secretary Scott McClellan said.
"The initial testing came back positive. There was some additional testing that was done and it was inconclusive," he said. "We're still waiting on more definitive results."
The Pentagon's mail delivery site, which is separate from the main Pentagon building, was evacuated and shut down Monday after sensors triggered an alarm around 10:30 a.m. EST, spokesman Glenn Flood said. It was expected to remain closed until at least Tuesday while the investigation continued.
While subsequent tests proved negative, the appearance of the bacteria is now the target of a criminal investigation by the FBI (search), Centers for Disease Control and Prevention (search) and local law enforcement. The substance will be further tested at a facility at Ft. Meade, Md. (search)
It was not clear when sensors at the second Defense Department mailroom were triggered Monday, and Pentagon officials only said a nearby satellite mail facility was closed. But firefighters in nearby Bailey's Crossroads, Va., reported that a military mailroom had been shut down after a hazardous material was detected, and no one was allowed to leave that building.
Pentagon spokeswoman Lt. Cmdr. Jane Campbell said mail at both facilities were irradiated before arriving at either one. The radiation treatment would kill any anthrax bacteria, but sensors would still be able to detect it.
About 175 people work at the Pentagon's mail facility, and another 100 may have been in contact with deliveries for the Pentagon, officials said.
Medical personnel took cultures from anyone who may have had contact with those deliveries, and those people were also offered a three-day course of antibiotics and told to watch for the signs of anthrax exposure: fever, sweats and chills.
Follow-up tests were being conducted at the U.S. Army Research Institute for Infectious Diseases (search) at Fort Dietrich, Md., officials said. They would take two to three days to complete.
General operations at the Pentagon appeared unaffected.
Anthrax can be spread through the air or by skin contact. Officials noted that sometimes anthrax sensors can give false-positive results.
Several cases involving letters laced with killer substances remain unsolved.
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.
In October 2003, two letters containing the poison ricin, sent to the Transportation Department and White House, were intercepted before they reached their destinations. The letters objected to new rules for long-haul truckers.
A small amount of ricin was discovered Feb. 2, 2004, on a mail-opening machine in the office suite of Senate Majority Leader Bill Frist, R-Tenn. The discovery led to a shutdown of three Senate office buildings for several days, and about two dozen staffers and Capitol police officers underwent decontamination.
The Postal Service has since installed anthrax detection equipment in mail-handling facilities across the country in hopes of detecting any future attack early and preventing spread of the agent.
Former government bioterror expert Stephen Hatfill (search) was under surveillance for months following the attacks and was described by the Justice Department as a "person of interest." He sued former Attorney General John Ashcroft and other government officials for unspecified monetary damages, saying his reputation was ruined.
No one has ever been charged with the crimes.
FOX News' Nick Simeone and The Associated Press contributed to this report.