Preliminary information indicates that some of the cultures coming back of samples taken from Washington, D.C., mail facilities are testing positive for anthrax, a senior health official told FOX News on Tuesday.

The tests follow the discovery of the potentially deadly bacteria last Thursday by sensors at two military mail facilities in the Washington area. The mail had been irradiated (search) before it reached those destinations, rendering any anthrax inert, defense officials told FOX News. The substance was discovered in a filter on the mail-scanning device.

The V Street post office in Washington also was closed as a precaution on Tuesday since officials said the mail forwarded to the Pentagon and another nearby Defense Department mailroom may have come from that post office. Environmental crews are determining whether the place was contaminated by anthrax.

D.C. Mayor Anthony Williams offered the 200 workers from that post office a three-day course of the antibiotic Cipro. The city's chief medical officer said no cases of the illness have been reported at 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," Mayor Anthony Williams (search) said.

Also on Tuesday, an Internal Revenue Service building was scoured after a letter containing a powdery substance was found. IRS officials said in a statement that "initial tests were negative for chemical or biological substances." Later, sources suggested the substance was rat poison.

Asked what the chances are of a false positive in the case of the tested culture, the source told FOX News that the likelihood is "low." Field tests often can come back as a false positive, but the secondary tests appear to confirm the DNA test taken at the scene.

Anthrax can be used as a biological weapon, but officials did not say whether the substance found demonstrated a terror attack.

The health official said the test demonstrates that the D.C. Health Department was clearly justified in taking the precautionary measures it did in the morning.

President Bush was being regularly updated of the situation as testing continued on Tuesday, White House press secretary Scott McClellan said.

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. The appearance of the bacteria made it the target of a criminal investigation by the FBI (search), Centers for Disease Control and Prevention (search) and local law enforcement.

Pentagon spokeswoman Lt. Cmdr. Jane Campbell said mail at both facilities was 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.

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.

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' Catherine Herridge and Nick Simeone and The Associated Press contributed to this report.