The US Postal Service has shut down a government mail handling office in Washington for anthrax testing.
At least five people died in anthrax attacks two years ago
Routine tests at the US navy mail office found traces of a substance that could be anthrax, said a Homeland Security department spokesperson.
Eleven postal offices in the Washington area have also been shut for further tests but it is not thought any postal worker has been exposed to the poison.
The naval office handles mail for federal agencies and the White House.
Air monitors at the office detected "small amounts of biological pathogens, possibly anthrax" on Wednesday, said Rachael Sunbarger, of the Homeland Security department.
Closer investigation of eight samples of the air revealed one positive test for anthrax. Further tests are now being carried out.
Naval staff said only a few people worked in the office. They would be tested as a precautionary measure, but were "very unlikely" to be at risk of exposure.
"Almost all the mail that's processed there is irradiated, so it's likely the substance would be inactive," said Lt Corey Shultz.
The Postal Service said it was shutting 11 offices in the Washington area that were served by the same contractor that transported mail to the navy centre.
"Out of an abundance of caution we want to make sure we've covered everything," said spokesman Mark Saunders. "We learned late Thursday the navy test was positive and that more samples were being taken at the facility".
Around 1,500 staff were affected and would be moved to other centres for the time being, he said.
At least five people died - including two Washington postal workers - and 17 were taken seriously ill within weeks of several anthrax attacks in October 2001.
Letters containing anthrax spores were sent to news media in New York and Florida, and anthrax-laced letters were sent to the Washington offices of two senators.
Those responsible have not yet been caught.
Since the attacks, the Postal Service has spent millions of dollars to test and install equipment to sterilise the post.