The female shooter was reported to be dead; it was not clear if she killed herself or was taken out by law enforcement. "Now barricaded inside a room with coworkers", YouTube employee Vadim Lavrusik wrote on Twitter before saying he was evacuated.
Brent Andrew, with San Francisco General Hospital, described the shooting as a "multiple casualty" event. Barberini said officers found a wounded victim outside of the building.
YouTube parent company Google tweeted: "YouTube situation, we are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available". A fourth victim was also transported to an area hospital.
Several YouTube employees tweeted Tuesday afternoon about chaos erupting across the campus after workers heard gunshots and reports of an active attack.
San Bruno Police tweeted at 1 p.m.to avoid the area due to police activity.
"It was very chaotic as you can imagine", San Bruno Police Department Chief Ed Barberini told reporters during a press conference at the scene.
Helicopters video showed a heavy concentration of police vehicles surrounding the YouTube building on Cherry Avenue in San Bruno.
Police on the scene said they have evacuated hundreds of people from the YouTube building.
Google, which owns the world's biggest online video website, posted on Twitter that the company is co-ordinating with authorities.
The San Francisco Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) said it was responding to the scene.
Will Hudson said a friend who works for YouTube texted him about the gunwoman.
The White House said President Donald Trump was briefed on a shooting and that officials were monitoring it. Heard shots and saw people running while at my desk.