A package that may contain Sarin has been found at Facebook HQ's mailing facility in Menlo Park, California.

The fire department has cordoned off the area and two people, who are feared to have been exposed to it, are being checked over.

Neither shows any signs that they have been poisoned. It remains unclear who the package was addressed to, if anyone.

The possible traces of Sarin were detected in a mailing facility that is used by Facebook in Menlo Park. The facility was evacuated afterwards

The alarm was raised by a machine which detected a possible trace of the highly toxic compound during a routine check on Monday.

As a precaution, the building was evacuated.

A Facebook spokesman told DailyMail.com: '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.'

The package was detected at the Facebook HQ (pictured) mailing facility on Monday

Firefighters wearing hazmat suits are now on the scene to ensure it is safe.

According to CBS, the machine conducted the test three times.

Twice, it detected the highly toxic compound and the test came back negative once.

Sarin is a highly dangerous nerve agent which can be deadly. It has been used as a weapon of war in the past, notably in Syria in 2017.

The colorless, odorless liquid disrupts the nervous system and eventually stops breathing.