The Postal Service has closed its government mail facility in Washington so experts can run anthrax tests.
The testing is being done to determine whether anthrax was detected at a Naval site that handles mail for federal agencies.
A Postal Service spokesman calls the testing at the facility, which has few workers, precautionary.
Equipment that routinely monitors the air at the Naval Automated Processing Facility, also located in the District of Columbia, indicated the presence of "small amounts of biological pathogens, possibly anthrax."
A Homeland Security spokeswoman says that happened Wednesday.
After the initial field test, one sample tested positive for anthrax and seven tested negative.
No illnesses have been reported.