Samples collected from a Defense Department mail center near the Pentagon building have tested positive for deadly anthrax bacteria in a preliminary examination, the U.S. government said on Tuesday.

Bill Hall, a spokesman for the Department of Health and Human Services, said an additional, more definitive test was being conducted on the sample to confirm whether it is anthrax. Results could be available later in the day.

Hall said federal health officials would send a bulletin to doctors, hospitals and clinics asking them to watch for patients suffering from symptoms of exposure to anthrax.

“It doesn’t mean we’re expecting anything out there, but it’s just out of the abundance of caution,” Hall said.

Two Defense Department mail-handling facilities were closed on Monday after sensitive detection devices indicated the presence of anthrax, but mail had already been irradiated to kill any bacteria, Pentagon officials said.

Anthrax is an acute, sometimes deadly infectious disease caused by a spore-forming bacterium that can be used as a biological weapon.

In 2001, five people died when anthrax was sent in letters to media and government offices in Washington, Florida and elsewhere, raising fears of bioterrorism. Those cases have not been solved.

On Tuesday, a Washington postal center also was closed and about 200 employees offered antibiotics as a precaution in case it had handled the Pentagon mail, officials said.

Hall said experts registered a positive indication for anthrax using a polymerase chain reaction (PCR) test on a sample taken from the Defense Department’s remote mail distribution facility, located just outside the Pentagon building in Arlington County, Virginia.

But Hall said the test was not considered definitive and the finding is considered preliminary.

“That doesn’t mean there is live anthrax and it doesn’t mean that there is actually any anthrax,” Hall said.

He said there were no results yet from either the preliminary or the definitive test of samples taken from the other facility closed on Monday - a separate satellite Defense Department mail center located in an office complex several miles away.

The Washington postal center that was closed on Tuesday was being screened for anthrax traces, said Dr. Gregg Pane, director of the District of Columbia’s Department of Health. He said there was “no confirmed detection” nor had employees reported any illness.

“This is a prudent course of action,” Pane said. “I don’t think there’s cause for alarm or panic or undue worry.”