Facebook has evacuated four buildings and two people are being evaluated for possible exposure to the nerve agent sarin after a package at the company’s Silicon Valley mail facility tested positive for the toxic compound.

The people who came in contact with the suspicious package about 11 am on Monday did not show any symptoms of exposure, said Jon Johnson, fire marshal for the city of Menlo Park, where Facebook is based.

“The facility tests all of the packages that come in and they had a positive test, so they just initiated their standard protocol,” he said.

“Now we’re just waiting to verify whether that’s true or not.”

Facebook spokesman Anthony Harrison said the company evacuated four buildings following the positive test and was co-operating with police.

“Authorities have not yet identified the substance found,” Harrison said.

“As of now, three of the evacuated buildings have been cleared for repopulation.”

Agents from the San Francisco office of the Federal Bureau of Investigation were responding to the scene, a spokesman for the agency said.

Sarin, a potent toxic compound that disrupts the nervous system, has been used as a chemical weapon.

Exposure can be fatal.

– AAP