Facebook Inc evacuated four buildings and two people were being evaluated for possible exposure to the nerve agent sarin on Monday after a package at the social media company's Silicon Valley mail facility tested positive for the toxic compound.

Facebook evacuates four buildings after possible sarin exposure

A bomb threat prompted authorities on Tuesday to evacuate a building at the Silicon Valley headquarters of Facebook, police said, but gave the "all clear" after an hours-long search turned up no sign of a device.

No suspicious package found in Facebook HQ at Menlo Park: Police

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Facebook evacuates four buildings after possible sarin exposure

No suspicious package found in Facebook HQ at Menlo Park: Police