At least three people were injured when a woman opened fire at YouTube headquarters before fatally shooting herself, local police said.
The female suspect died in a possible self-inflicted gunshot wound, they said, adding that one of the victims, a 36-year-old man, is said to be in a critical condition.
The YouTube headquarters in San Bruno near Silicon Valley has a significant number of Indian-American professionals.
In a tweet, US President Donald Trump said he was briefed on the shooting.
"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," he said.
It was around 12.46 pm (local time) that the San Bruno Police Department received numerous 911 calls reporting gunfire at the YouTube campus located at 901 Cherry Avenue in San Bruno. The police arrived at the scene at 12:48 pm, two minutes after the first call, and immediately began searching for a possible suspect.
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Arriving officers encountered numerous employees fleeing the building. Officers discovered one victim with an apparent gunshot wound to a lower extremity at the front of the building.
At 12:53 pm, officers conducting a search located a deceased female inside with a possible self-inflicted gunshot wound.
"We have one subject who is deceased inside the building with a self-inflicted wound at this time, (who) we believe to be the shooter," San Bruno Police Chief Ed Barberini told reporters at a news conference outside YouTube headquarters.
Two additional victims were located after fleeing to a next-door business.
"A total of four people were transported to local hospitals, three of which had gunshot wounds. The extent of the victims' injuries is unknown at this time," the police said.
Google, which owns the world's biggest online video website, in a tweet, advised all other employees in the Bay Area, and people with meetings scheduled, to stay away from the area.
"There is no need to take any action. We have provided employees a helpline," it said.
In another tweet, it said the company was coordinating with authorities.
"Re: YouTube situation, we are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available," Google communications tweeted.
A law-enforcement source told the Bay Area News Group that the shooting appeared to have been fuelled by a domestic dispute and that the suspect was targeting her boyfriend.
Apple CEO Tim Cook sent his sympathies to the victims.
"From everyone at Apple, we send our sympathy and support to the team at YouTube and Google, especially the victims and their families," Cook tweeted.
"My stomach sinks with yet another active shooter alert. I'm praying for the safety of everyone at YouTube headquarters," tweeted Senator Dianne Feinstein.
YouTube headquarters is located some 50 kilometres from the main Google campus in Mountain View.
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