Officers walk near a YouTube office in San Bruno, Calif., Tuesday, April 3, 2018. Police in Northern California are responding to reports of a shooting at YouTube headquarters in the city of San Bruno.

Image Credit: AP Photo/Jeff Chiu

SAN BRUNO, Calif. - A woman opened fire Tuesday at YouTube headquarters, wounding three people before fatally shooting herself as terrified employees huddled inside, police and witnesses said.

After receiving multiple 911 calls reporting gunfire, 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 gun shots, 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 a male employee repeatedly called 911 for updates.

"It was terrifying," she said.

Television news footage showed people leaving the building in a line, holding their arms in the air. Officers patted them down to make sure none had weapons.

Officers discovered one victim with a gunshot wound when they arrived and then found the shooter with what appeared to be a self-inflicted gunshot wound several minutes later, San Bruno Police Chief Ed Barberini said.

He said two additional gunshot victims were later located at an adjacent business and that a fourth injured person suffered an ankle injury.

The 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 several years ago famously outfitted the office with a 3-lane red slide for workers to zoom from one story to another.

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 on the campus 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.'"

There was somebody lying nearby on his back with a red stain on his stomach that appeared to be from a bullet wound.

He said he realized it was an active shooter incident when a police officer with an assault rifle came through a security door. He jumped on his skateboard and ran away.

San Francisco General Hospital received three patients: a 36-year-old man in critical condition, a 32-year-old woman in serious condition and a 27-year-old woman in fair condition, a spokesman said.

The hospital said later that it did not expect to receive more patients.

Google, which owns the world's biggest online video website, posted on Twitter that the company was co-ordinating with authorities. The Bureau of Alcohol, Tobacco, Firearms and Explosives said it also responded.

The White House said President Donald Trump was briefed on a shooting and that officials were monitoring developments.

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Associated Press writer Michael Balsamo in Los Angeles contributed to this report.

News from © The Associated Press, 2018