Tests on filters and equipment at two shut-down postal centers that serve the Pentagon have turned up no evidence of deadly anthrax, the Defense Department’s top health official said on Tuesday.

Assistant Defense Secretary for Health Affairs William Winkenwerder said these tests provided “reassuring news,” but stopped short of declaring the situation a false alarm pending the completion of additional tests.

“We have no evidence that there was anthrax material in the mail,” Winkenwerder told reporters.

Winkenwerder noted that an initial test came back as positive for the anthrax bacterium at a mail-processing facility located on the grounds of the Pentagon, the colossal headquarters for the U.S. military.

“Our subsequent test results in and around the area where that positive initial test result was obtained, as well as many other areas, have all come back negative,” Winkenwerder said.

“Based on all this, we would say at this time that the probability is low to very low that we’re dealing with a true health threat. However, we want to continue to perform testing and that will be going on throughout the night and tomorrow (Wednesday),” he added.

He said he hoped the Pentagon could make “an absolutely definitive statement” on Wednesday.

Two Defense Department mail-handling facilities in Virginia - the one next to the Pentagon and another several miles away - were closed and hundreds of workers evacuated on Monday after sensitive detection devices indicated the presence of anthrax, Pentagon officials said. But the mail had already been irradiated to kill any bacteria, the 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.

Winkenwerder said the two Virginia facilities remained closed, but said it was possible they could be re-opened as earlier as Wednesday morning.

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

Winkenwerder said experts initially 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.

The facility is intended to screen everything coming into the Pentagon, from letters to packages and construction materials.

He said subsequent tests on filters at both facilities as well as mail-processing equipment came back negative.

Earlier, Bill Hall, a spokesman for the Department of Health and Human Services, 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.

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 health department. He said there was “no confirmed detection” nor had employees reported any illness.

Glenn Flood, a Pentagon spokesman, said the mail room samples that first led sensors to detect anthrax at the two Pentagon postal facilities were collected last Thursday, with the results confirmed on Monday.