The US Postal Service has shut down 11 postal centres in the Washington area after tests indicated possible anthrax at a US Navy mail handling centre.
The navy closed the automated mail handling operation at its naval air station in Washington on Thursday to run additional tests after sensors detected traces of a substance that could be anthrax, a navy spokeswoman said.
The substance was identified late on Wednesday by equipment that routinely samples the air in the facility.
"Almost all the mail that's processed there is irradiated, so it's likely the substance would be inactive," the spokeswoman said. She also noted that it was "very unlikely" the few people who work in the facility were at risk of exposure.
As a precaution, the U.S. Postal Service closed 11 facilities that were serviced by the same contractor that transported mail to the navy centre, a spokesman said.
In 2001 a series of anthrax attacks, which spanned the East Coast from Florida to Connecticut, killed at least five people, including two Washington postal workers.
Letters containing anthrax spores were mailed to news media offices in New York and Florida and anthrax-laced letters were sent to the Washington offices of two senators.
Since the anthrax attacks, the Postal Service has spent millions of dollars to test and install equipment to sterilise mail.