Facebook employees have been evacuated from part of the company's sprawling headquarters campus due to a bomb threat.
Police in Menlo Park, California published an alert on Tuesday evening telling members of the public to avoid the area.
Bomb disposal units, it said, were "going through the building", and video from a local TV chopper showed police vehicles and a fire engine clustered in the campus car park.
The threat was originally called in to police in New York City, via a phone line offering cash rewards for anonymous information, and then referred to the Menlo Park police department.
Nicole Acker, a spokeswoman for the Menlo Park police, said the evacuation was confined to a three-story facility on campus that she said was not the headquarters building, but a company spokesman said by email that "a few" buildings on the site had been evacuated.
The building that was evacuated also houses employees of Instagram, which is owned by Facebook.