The people who came in contact with the suspicious package at about 11 a.m. PT (1800 GMT) did not show any symptoms of exposure to sarin, said Jon Johnston, fire marshal for the city of Menlo Park, California, 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.

Fire personnel were preparing to enter the mail facility at around 6 p.m. to assess the packages, Johnston 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. (Reporting by Katie Paul in San Franciso; Andrew Hay in New Mexico and Dan Whitcomb in Los Angeles; Editing by Richard Chang and Lisa Shumaker)