A woman opened fire with a handgun at YouTube's global headquarters in Silicon Valley on Tuesday, shooting three people before killing herself.
The woman approached an outdoor patio and dining courtyard on the campus around lunchtime and began to fire before entering the building of the Google-owned video sharing service.
The shooter was identified by police officials as Nasim Aghdam of Southern California, US media reported.
Sources told NBC that Aghdam appeared to have a YouTube channel and had posted videos criticising the video-sharing service for censorship.
According to the broadcaster, she says in a video posted in January 2017 that YouTube “discriminated and filtered” her content. In the video, Aghdam reportedly says her channel used to get lots of views but that after being “filtered” by the company, it received far fewer views.
Police have yet to officially the name the suspect.
Chief Ed Barberini of San Bruno police said the three victims, a man and two women, suffered from "treatable injuries". Two were shot "at an adjacent business".
Hospital officials have said the 36-year-old man is in critical condition and two women, aged 32 and 27, are in serious and fair condition. A fourth person hurt suffered a sprained or broken ankle.
The police chief, who said the weapon used was a handgun, said the first 911 calls came at 12:46pm. His officers were on the scene two minutes later.
“Upon arriving, officers found a chaotic situation with employees streaming out of the building,” he said.
“We did encounter one victim with apparent gunshot wound towards the front of the building as we arrived.
“Several minutes later, while conducting a search of the premises, officers located a second individual with a gunshot wound that appeared to be self-inflicted.
“There were two more at an adjacent business.”
An employee at a nearby Carl's Jr. fast food restaurant said one of the victims came in after being shot.
The employee told KTVU the female victim had a gunshot wound to her calf and he tied a makeshift tourniquet around it.
None of the victims were named last night, but Fox News reported that one of the victims was the shooter's boyfriend, and police were not treating the incident as terrorism.
Mass shootings by women are exceptionally rare.
A FBI study of shootings from 2000 to 2013 found that, of 160 incidents, only six were carried out by a female attacker.
President Donald Trump said that he was monitoring events, and wrote on Twitter: "Was just briefed on the shooting at YouTube’s HQ in San Bruno, California. Our thoughts and prayers are with everybody involved. Thank you to our phenomenal Law Enforcement Officers and First Responders that are currently on the scene."
Sundar Pichai, Chief Executive Officer of Google, wrote to staff saying "the best information we have right now is that the situation is contained".
"I know a lot of you are in shock right now," he wrote. "Over the coming days, we will continue to provide support to help everyone in our Google family heal from this unimaginable tragedy."
Shaken YouTube employees were left reeling from the incident, which was over in less than an hour.
Todd Sherman, a product manager in the building, wrote on Twitter: "We were sitting in a meeting and then we heard people running because it was rumbling the floor. First thought was earthquake."
On the way out of the building he "looked down and saw blood drips on the floor and stairs".
He added: "Police cruisers pulled up, hopped out with rifles ready and I told them where the situation was."
Vadim Lavrusik, a YouTube employee, tweeted that he and his colleagues were inside the offices and saw people running and barricaded themselves inside the offices.
After around 20 minutes he tweeted again to say that he had been evacuated. His account was then hacked, with the perpetrator posting misinformation about the shooting.
Footage from helicopters showed hundreds of employees leaving the sprawling campus, being checked for weapons as they went.
“I was on a video conference with someone in the building when it happened,” one woman, who did not want to be identified, told CNN.
“We were suddenly aware of people running and screaming. The people on the video conference became scared and said: ‘I have to get out of here.’
“We called security, who had already been made aware of the incident. We heard on the video conference sounds of people running and screaming, but it was hard to hear anything else at that time.”
The woman said her colleague later told her she did not hear any shots.
A female bystander told KTVU television: "I heard boom, boom, boom. Then I heard it again. It was loud and then like it was going away."
Breaking down, she added: "I'm having a hard time. Don't get me started on guns."
In a statement Google, which owns YouTube, said: "Regarding the YouTube situation, we are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available."
The San Bruno offices house 1,700 employees in a largely open-plan environment, in four different buildings.
There is security at all the buildings, the employee told CNN.
YouTube announced last month that it would ban content promoting the sale of guns and gun accessories as well as videos that teach how to make guns.