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Two law enforcement sources told ABC News that authorities have preliminarily identified the YouTube shooter as Nasim Aghdam, with previous addresses in both Riverside and San Diego.
The officials with knowledge of the investigation spoke to The Associated Press on the condition of anonymity because they were not authorized to discuss it.
The shooter killed herself after opening fire with a handgun in a courtyard, police 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, as terrified employees huddled inside.
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.
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.
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 wounded victim 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 found minutes later at an adjacent business. Barberini initially said there were four shooting victims but later clarified that a fourth 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, which owns the world's biggest online video website, famously outfitted the office several years ago with a three-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 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 there was an active shooter when a police officer with an assault rifle came through a security door. He jumped on his skateboard and took off.
Google said the company's security team worked with authorities to evacuate buildings and was doing whatever it could support the victims and their families. The Bureau of Alcohol, Tobacco, Firearms and Explosives said it also responded.
"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.
Trump said Tuesday on Twitter that he was briefed on the shooting in San Bruno, California. He wrote, "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."
This story has been corrected with new information from authorities that three victims suffered gunshot wounds and a fourth was taken to the hospital but wasn't shot. A reference also has been removed to patients being taken to Stanford Hospital because the facility says it gave incorrect information.