Anthrax tests from two Pentagon (search) mailrooms came back negative Tuesday, a day after initial testing indicated the deadly spores might be present, prompting nearly 900 workers to take antibiotics as a precaution.

Responding to what now appear to have been false alarms, officials handed out antibiotics and closed three mail facilities — two that serve the Pentagon and one in Washington that handles mail on its way to the military.

Officials believe that the confusion stemmed from a mistake at a Defense Department laboratory at Fort Detrick, Md. Officials there apparently mixed up a sample of actual anthrax that is kept on hand for comparison purposes with the sample taken from the Pentagon mailroom, a senior administration official said Tuesday.

Later tests proved negative and officials realized their error, the official said, speaking on condition of anonymity.

"We had some preliminary results that were positive but subsequent additional tests have determined that the sample that we had was in fact negative," said Dr. William Winkenwerder (search), assistant secretary of defense for health affairs.

He said tests that have been completed on samples from both facilities have all come back negative, though some additional tests are still incomplete.

"So on that basis we have nothing to suggest anything remotely like the events of October 2001, and we hope that with further information we'll be able to completely rule out any threat at all," he said.

In more than three years since the 2001 anthrax-by-mail attacks, there have been scores of initial tests that falsely reported anthrax (search) in government mailrooms. In this case, however, two alert systems independently suggested the presence of the bacteria, raising concerns and invoking memories of the attacks that killed five and panicked Americans still raw from the Sept. 11 attacks.

Officials became concerned after warning signs of anthrax appeared at two Pentagon mail facilities on Monday, in what appears now to have been a coincidence. First, a filter on a device that screens mail for chemical and biological agents on the Pentagon grounds tested positive for anthrax. Separately, an alert was set off at a nearby satellite mail processing facility.

After receiving results that appeared to confirm the initial readings, officials set out to retest the initial filter and gathered additional samples from the facilities for testing. Every one of those samples came back negative, Winkenwerder said.

"We're very encouraged with the information that we now have in hand," he said.

As a precaution, antibiotics were given to 166 employees at a post office processing center in the District of Columbia, which handles mail before it reaches the Pentagon, and to about 700 workers at the military mailrooms, officials said. That includes those at the facility on the Pentagon grounds in Arlington, Va., and those who work at the satellite facility several miles away in Fairfax County, Va.

Winkenwerder said the advice to these workers would not change until the final tests come back, probably Wednesday. Assuming those tests are also negative, any workers who began taking antibiotics would be advised to stop.

During the course of a day that had many on edge, hospitals were told to be on the lookout for symptoms including respiratory problems, rashes or flu-like symptoms that could signal exposure to anthrax, which can be used as a biological weapon.

Anthrax can be spread through contact with the skin. A more serious form of the disease, inhalation anthrax (search), is contracted by breathing in spores. After the 2001 attacks, health officials concluded that some people can contract the disease through exposure to a small number of the tiny microbes.