Update - 10.43pm: The Chief of Police at San Bruno, Ed Barberini, says they have found one dead person, said to be female, who appears to have shot themselves.
She is believed to be the shooter at YouTube's California headquarters, but it has not been confirmed as yet.
He said they also found four people with "gunshot-related injuries" at the scene.
“We have four victims who have all been transported for gunshot-related injuries and we have one subject who is deceased inside the building with a self-inflicted wound who, at this time, we believe to be the shooter but we are still following up on that,” he said.
Police activity at 901 Cherry Ave, please stay out of the area. pic.twitter.com/H6iAj0g7ra— San Bruno Police (@SanBrunoPolice) April 3, 2018
He went on to say that it is an active event and an active investigation.
Brent Andrews from the Stanford General Hospital has said that they have taken in three patients from the shooting, one of whom has been described as critical.
He said a 36-year-old man is in critical condition, a 32-year-old woman is in serious condition and a 27-year-old woman is in fair condition.
He said the hospital was expecting more patients but does not know their conditions.
YouTube product manager Todd Sherman said he was in a meeting when he heard people running for the exit.
He said it “seemed serious and not like a drill.”
10.23pm: Hospital takes in up to five patients as California police hunt 'active shooter' at YouTube campus
A spokeswoman for California's Stanford Hospital said it had received four to five patients from the shooting at YouTube but had no information about their condition.
San Bruno police said they could not provide more information about the incident.
The White House said President Donald Trump had been briefed on the shooting.
YouTube employee Vadim Lavrusik posted on Twitter that he heard gunshots and saw people running.
He said he was barricaded in a room with his co-workers before being safely evacuated.
Active shooter at YouTube HQ. Heard shots and saw people running while at my desk. Now barricaded inside a room with coworkers.— Vadim Lavrusik (@Lavrusik) April 3, 2018
Will Hudson said his friend who works for YouTube texted him about the assailant.
"I think there might be a shooter in my building," read one text.
"The fire alarm went off so we started to evacuate and then people (started) running saying there was a shooter."
Mr Hudson said his friend has made it safely back to San Francisco and is in contact with his family.
Mr Hudson said he had become accustomed to hearing about such violence but has never been so close to it before.
"It just feels strange. It feels like it could really be anyone. That's really the strangeness of it," he said.
9.50pm: California police hunting 'active shooter' at YouTube campus
Police in California are responding to reports of a shooting at YouTube headquarters in the city of San Bruno.
City manager Connie Jackson said there were multiple 911 calls reporting a shooting and that police and fire officials have responded.
Police tweeted to warn people to stay away from Cherry Avenue and Bay Hill Drive as the incident unfolded.
TV helicopter video shows officers searching people grouped outside a building and police vehicles surrounding the area.
San Bruno police said they could not provide more information.
The world's biggest online video website is located on a suburban campus.
Calls and emails to YouTube representatives were not immediately returned.
Google Communications tweeted in light of the YouTube shooting reports to say it was coordinating with authorities and will provide official information as it becomes available.