WASHINGTON (Reuters) -- The U.S. Postal Service shut down 11 postal facilities in the Washington area late on Thursday after preliminary tests indicated possible anthrax at a U.S. Navy mail handling center, officials said.

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 and preliminary tests indicated that it could possibly be anthrax, a deadly bacterial disease, Lt. Corey Schultz said. More definitive testing was underway, she added.

"Almost all the mail that's processed there is irradiated, so it's likely the substance would be inactive," Schultz said. She also noted that it was "very unlikely" that the few people who work in the facility were at risk of exposure, but said that they were being evaluated as a precautionary measure.

As a precaution, the U.S. Postal Service closed 11 facilities that were serviced by the same contractor that transported mail to the Navy center, a spokesman said. "Out of an abundance of caution we want to make sure we've covered everything," spokesman Mark Saunders said. "We learned late Thursday the Navy test was positive and that more samples were being taken at the facility."