The deadly poison sarin was detected in a mail bag at Facebook's offices in California

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

On Monday, routine tests for sarin detected the presence of the substance in a mail bag 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 currently 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.

Sarin is a man-made nerve agent used in chemical warfare. Exposure can cause loss of consciousness, convlusions, paralysis, respiratory failure, and death, according to the CDC.

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

SF Gate reported that two buildings at Facebook's headquarters have been evacuated. A Facebook spokesperson did not immediately respond to Business Insider's request for comment.

This story is developing...