Eleven U.S. post offices remained closed Friday while authorities ran tests to determine whether anthrax was found at a Navy postal site.

Washington's Homeland Security office said air-monitoring equipment detected small amounts of biological pathogens, possibly anthrax, at the facility that handles mail for government agencies.

A spokesman for the Naval District of Washington said one sample tested positive for anthrax and seven tested negative.

The one test showed a spore count of 138. Someone would have to breathe 8,000 to 10,000 spores to become infected.

No one has been hospitalized and no indication any postal workers were exposed.

Five Americans died in a series of anthrax attacks two years ago.

A month after the Sept. 11 strikes, anthrax-laced letters arrived at offices of two U.S. senators and to news media offices in New York and Florida.