The Deadly Poison Sarin Was Detected in a Mailbag at Facebook's Offices in California Workers at the facility are now being monitored, but none have so far shown any symptoms of exposure, officials said.

The deadly poison sarin has been detected at a mail facility for Facebook in Menlo Park, California, prompting building evacuations and FBI involvement.

On Monday, routine tests for sarin detected the presence of the substance in a mailbag at the social-media tech firm's offices, a Menlo Park Fire Department (MPFD) official told Business Insider. The mailbag is now under quarantine. The news was first reported by NBC Bay Area.

It is possible that the detection is a false positive, the official said.

A number of people who work at the facility are now being monitored, but no one is showing any symptoms of exposure, according to officials. The MPFD official could not say exactly how many people were being monitored, though other news reports have suggested two people.

A Facebook spokesperson said the company evacuated four buildings in response to the news, three of which have now been brought back into use.

Sarin is a man-made nerve agent used in chemical warfare. Exposure can cause loss of consciousness, convulsions, paralysis, respiratory failure, and death, according to the Centers for Disease Control and Prevention.

Law enforcement has been notified and the FBI is en route.

In a statement, the Facebook spokesperson Anthony Harrison said: "At 11:00 AM PDT this morning, a package delivered to one of our mail rooms was deemed suspicious. We evacuated four buildings and are conducting a thorough investigation in coordination with local authorities. Authorities have not yet identified the substance found. As of now, three of the evacuated buildings have been cleared for repopulation. The safety of our employees is our top priority and we will share additional information when it is available."

This story is developing ...