Washington ? Anthrax tests from two Pentagon 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, 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 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 9-11 attacks.

U.S. Postal Service employees sit in the waiting area of D.C. General Hospital before being screened for anthrax contamination in Washington. Tests on Tuesday showed that a substance believed to be anthrax was not.

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.

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. .