A Facebook mail facility near the company’s California headquarters was evacuated after a routine check found mail possibly containing the nerve agent sarin.

Authorities put the Menlo Park site under quarantine as they conducted additional testing. Four buildings were evacuated and three have been cleared for people to go back in, said spokesman Anthony Harrison.

The suspicious package was delivered at around 11am to one of the company’s mail rooms, he said.

“Authorities have not yet identified the substance found,” Mr Harrison wrote.

The company's headquarters in Menlo Park
The company’s headquarters in Menlo Park (Marcio Jose Sanchez/AP)

There were no reports of injuries, Menlo Park Fire Marshal Jon Johnston said.

Incoming mail undergoing routine processing by machine tested positive for sarin, but it could have been a false positive, Mr Johnston said.

“Right now we don’t have anybody that has any symptoms,” he said. “We’re just doing verification.”

The FBI is assisting in the investigation, as is common in incidents such as this one.

The federal Centres for Disease Control and Prevention says sarin is a chemical warfare agent that is a clear, colourless, odourless and tasteless liquid. It can evaporate into the environment, prompting symptoms within seconds.

A drop of sarin on skin can cause sweating and muscle twitching, and exposure to large doses can result in paralysis and respiratory failure leading to death.

The CDC says people who are mildly exposed usually recover completely.