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Officials say three people were wounded by gunshots, and another person suffered a twisted ankle.
San Bruno Police Chief Ed Barberini initially told reporters Tuesday that four people were wounded. He later clarified that one person who was taken to the hospital had a twisted ankle or similar injury but wasn't shot.
Two federal law enforcement officers tell ABC News that early indications point to the shooting being the result of a domestic situation. It does not appear to be terror-related.
Officers and federal agents swarmed the company's headquarters complex in the city of San Bruno as multiple 911 calls came in reporting gunfire at 12:46 p.m.
PHOTOS: Powerful images from shooting at YouTube HQ in San Bruno
The City of San Bruno Police Department issued a statement Tuesday afternoon, confirming a total of four people were taken to local hospitals. Three of those injured had gunshot wounds.
Brent Andrew, a spokesperson at San Francisco General Hospital, says they're treating three people - a 36-year-old man in critical condition and two women, ages 32 and 27, who are in serious and fair condition.
San Bruno Police Chief Ed Barberini said one victim was found near the entrance, and two others were injured at a nearby business.
SKY7 was over the campus, where law enforcement was seen approaching the building with guns drawn. About 200 employees were evacuated with arms overhead, and the campus was flanked with dozens of police cars and emergency vehicles. Officers patted down people to make sure none had weapons.
VIDEO: Aerial footage shows active shooting situation at YouTube HQ
About 1,700 people work at the campus.
Police activity at 901 Cherry Ave, please stay out of the area. pic.twitter.com/H6iAj0g7ra— San Bruno Police (@SanBrunoPolice) April 3, 2018
Google, which owns the world's biggest online video website, posted on Twitter that the company is coordinating with authorities. The Bureau of Alcohol, Tobacco, Firearms and Explosives also said it responded to the shooting at YouTube's suburban campus.
Re: YouTube situation, we are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available.— Google Communications (@Google_Comms) April 3, 2018
The White House said President Donald Trump was briefed on a shooting and that officials were monitoring it.
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.— Donald J. Trump (@realDonaldTrump) April 3, 2018
Andre Campbell, a trauma surgeon who treated the victims at the hospital, said the shooting was another "sad day" for America.
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"This is unfortunate, and it continues," he said, referencing the Parkland, Las Vegas and Pulse nightclub shootings.
Hudson said his friend made it safely back to San Francisco and was in contact with his family. Hudson said he's become used to hearing about gun 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.
Officials say they will be at YouTube's headquarters through the night as their investigation continues.
Click here for full coverage on the shooting at YouTube's headquarters.
Associated Press reporters Jeff Chiu and Sudhin Thanawala contributed to this report