A woman who had voiced complaints online about YouTube has opened fire with a handgun at the tech company's headquarters near San Francisco, wounding three people before shooting herself dead, according to local authorities and media.

Key points: San Francisco General Hospital's spokesman said of the three being treated, one was in a critical condition

San Francisco General Hospital's spokesman said of the three being treated, one was in a critical condition Police confirmed there is "no immediate threat to the community"

Police confirmed there is "no immediate threat to the community" YouTube product manager Todd Sherman said he saw "blood drips on the floor and stairs" as he evacuated

The woman, 39-year-old San Diego resident Nasim Najafi Aghdam, approached an outdoor patio and dining courtyard on the YouTube campus around lunchtime and began to fire before entering the building, police said.

San Bruno police said in a statement that the three people who were shot at YouTube's headquarters in California were not specifically targeted.

"The San Bruno Police Department is investigating a motive for this shooting," the police statement said.

Nasim Aghdam had openly criticised what she said was YouTube's censorship. ( Supplied: San Bruno Police Department )

"At this time there is no evidence that the shooter knew the victims of this shooting or that individuals were specifically targeted."



Ms Aghdam's father, Ismail Aghdam told the Bay Area News Group he warned police his daughter might be going to YouTube because she "hated" the company.

Mr Aghdam said he reported his daughter missing on Monday after she did not answer her phone for two days.

He said the family received a call from Mountain View police early on Tuesday morning telling them they found Ms Aghdam sleeping in a car and he warned them she might go to YouTube.

Mountain View Police spokeswoman Katie Nelson confirmed officers located a woman by the same name in a vehicle asleep in a Mountain View car park on Tuesday morning.

She said the woman declined to answer further questions.

Ms Nelson did not respond to a question about whether police were warned Ms Aghdam might go to YouTube.

Sorry, this video has expired California police confirm YouTube shooter is deceased

The website NasimeSabz.com, which local media said was linked to the attacker, had several posts about Persian culture and veganism, interspersed with rants against YouTube.

Those complaints included claims the company was not sharing enough revenue with people who create videos for the platform.

"There is no equal growth opportunity on YouTube, or any other video-sharing site, your channel will grow if they want to," one posting on the site said.

'I looked down and saw blood drips on the stairs'

Officers respond to reports of an active shooter at a Youtube office in San Bruno. ( AP: Jeff Chiu )

A YouTube product manager, Todd Sherman, described on Twitter hearing people running, first thinking it was an earthquake before he was told that a person had a gun.

"At that point every new person I saw was a potential shooter. Someone else said that the person shot out the back doors and then shot themselves," Mr Sherman said in a tweet.

"I looked down and saw blood drips on the floor and stairs. Peeked around for threats and then we headed downstairs, and out the front," Sherman said.

In a recording of a 911 call posted online by the Los Angeles Times, a dispatcher can be heard saying: "Shooter. Another party said they spotted someone with a gun. Suspect came from the back patio … Again we have a report of a subject with a gun. They heard seven or eight shots being fired."

Dozens of emergency vehicles quickly converged on the YouTube campus, and police could be seen on televised aerial video systematically frisking several employees leaving the area with their hands raised.

One victim, a 36-year-old man, was listed in critical condition at San Francisco General Hospital.

A 32-year-old woman was listed in serious condition and a 27-year-old woman in fair condition. Authorities did not release names of the victims.

The three patients taken to San Francisco General Hospital were all awake, Andre Campbell, a trauma surgeon at the hospital, told a news conference.

All were victims of gunshot wounds, Dr Campbell said, but none of them had undergone surgery.

A fourth person was taken to a local hospital with an ankle injury suffered while fleeing the scene.

US President Donald Trump said on Twitter he had been briefed on the shooting.

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"Our thoughts and prayers are with everybody involved," Mr Trump tweeted.

"Thank you to our phenomenal law enforcement officers and first responders that are currently on the scene."

In response, Twitter chief executive Jack Dorsey tweeted: "We can't keep being reactive to this, thinking and praying it won't happen again at our schools, jobs, or our community spots. It's beyond time to evolve our policies."

The attack is the latest in a string of mass shootings in the US in recent years.

Most recently, the massacre of 17 people at a Florida high school has led to calls for tighter curbs on gun ownership.

Last month, YouTube announced it would ban content promoting the sale of guns and gun accessories, as well as videos that teach people how to make guns.

Authorities respond to reports of an active shooter at YouTube's headquarters. ( Reuters )

AP/Reuters