A woman opened fire with a handgun in a courtyard atYouTube headquarters in California, wounding three people before fatally shooting herself in what is being investigated as a domestic dispute, authorities said.
Terrified employees huddled inside, calling 911, as officers and federal agents swarmed the company's suburban campus sandwiched between two interstates in the San Francisco Bay Area city of San Bruno.
YouTube employee Dianna Arnspiger said she was on the building's second floor when she heard gunshots, ran to a window and saw the shooter on a patio outside.
She said the woman wore glasses and a scarf and was using a "big huge pistol."
"It was a woman and she was firing her gun. And I just said, 'Shooter,' and everybody started running," Arnspiger said.
She and others hid in a conference room for an hour while another employee repeatedly called 911 for updates.
"It was terrifying," she said.
Authorities gave few details about the shooter and victims, except that the gunfire was being investigated as a domestic dispute, two law enforcement told The Associated Press.
A 36-year-old man was in critical condition, a 32-year-old woman was in serious condition and a 27-year-old woman was in fair condition, a spokesman for San Francisco General Hospital said.
Witnesses described terror before officers arrived and discovered a victim near a front door and then found the shooter several minutes later with what appeared to be a self-inflicted gunshot wound, San Bruno Police Chief Ed Barberini said.
He said two additional gunshot victims were found at an adjacent business minutes later. Mr Barberini initially said there were four people who had been shot but later clarified that a fourth person had suffered an ankle injury.
Zach Vorhies, 37, a senior software engineer at YouTube, said he was at his desk working on the second floor of one of the buildings when the fire alarm went off. He got on his skateboard and approached a courtyard, where he saw the shooter yelling, "'Come at me, or come get me'."
He saw somebody lying nearby on his back with a red stain on his stomach that appeared to be from a bullet wound. Mr Vorhies said he realised there was an active shooter when a police officer with an assault rifle came through a security door.
He said the public can access the courtyard where he saw the shooter during work hours.
Michael Finney, a 21-year-old supervisor at Carl's Jr across from the campus, said he came out of the bathroom to see a woman in a booth bleeding from the calf. Two friends were trying to stop the bleeding, using the victim's sweatshirt as a tourniquet, but it was not helping, he told the San Jose Mercury News.
"Everyone was figuring out what to do," Mr Finney said. "I was trying to stay calm and see what I could do. Everybody is shocked."
Google, which owns the world's biggest online video website, said the company's security team worked with authorities to evacuate buildings and was doing whatever it could support the victims and their families.
YouTube's headquarters has more than a thousand engineers and other employees in several buildings. Originally built in the late 1990s for the clothing retailer Gap, the campus south of San Francisco is known for its sloped green roof of native grasses.
Inside, Google famously outfitted the office several years ago with a three-lane red slide for workers to zoom from one story to another.
"Today it feels like the entire community of YouTube, all of the employees, were victims of this crime," said Chris Dale, a YouTube spokesman. "Our hearts go out to all those who suffered."
The White House said President Donald Trump was briefed on the shooting and that officials were monitoring developments.