Police in Northern California say they are responding to an incident involving an “active shooter” at YouTube headquarters.
Gunshots were reported at the company’s campus in San Bruno, south of San Francisco.
Several people have been taken to hospital but their condition is unknown.
Google, which owns the video sharing website, has said it is investigating a possible “gunfire incident”.
Unconfirmed reports from local media say the shooter is a woman.
Ambulances are at the scene and authorities have warned the public to stay out of the area.