YouTube employees have reported that an active shooter is on the premises of their San Bruno headquarters in California.
Unconfirmed reports say it was a female shooter and that two people have been injured but police are yet to give any information.
Employee Vadim Lavrusik tweeted at around 1pm on Tuesday: 'Active shooter at YouTube HQ. Heard shots and saw people running while at my desk.
'Now barricaded inside a room with coworkers.'
Lavrusik was evacuated 20 minutes later.
Startling photographs from its expansive campus show employees leaving the building with their hands in the air.
Police say the situation is a 'very active' scene. San Bruno is around 11 miles south of San Francisco. Authorities have received 50 separate 911 calls of shots being fired on the premises.
The campus is spread over 200,000 sq ft and is where 1,700 employees work.