MENLO PARK, California — A Facebook mail facility near company headquarters was evacuated Monday after a routine check found mail possibly containing the nerve agent sarin.

Authorities put the site under quarantine as they conducted additional testing. Four buildings were evacuated and three have been cleared for people to come back in, said Facebook spokesman Anthony Harrison in a statement. The suspicious package was delivered around 11 a.m. to one of the company's mail rooms, he said.

"Authorities have not yet identified the substance found," Harrison wrote.

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, 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 Centers for Disease Control and Prevention says sarin is a chemical warfare agent that is a clear, colorless, odorless 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.