SAN FRANCISCO (Reuters) – Facebook Inc evacuated four buildings and two people were being evaluated for possible exposure to the nerve agent sarin on Monday after a package at the social media company’s Silicon Valley mail facility tested positive for the toxic compound.

The people who came in contact with the suspicious package at about 11 a.m. did not show any symptoms of exposure to sarin, said Jon Johnson, fire marshall for the city of Menlo Park, where Facebook is based.

“The (Facebook) facility tests all of the packages that come in and they had a positive test, so they just initiated their standard protocol. Now we’re just waiting to verify whether that’s true or not,” he said.

Facebook spokesman Anthony Harrison said the company evacuated four buildings following the positive test and was cooperating with police in the investigation.

“Authorities have not yet identified the substance found. As of now, three of the evacuated buildings have been cleared for repopulation,” Harrison said.

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.