SAN BRUNO: A woman opened fire with a handgun at YouTube's headquarters near San Francisco on Tuesday, wounding three people before shooting herself dead as employees of the Silicon Valley tech company fled into the surrounding streets, authorities said.

Police did not identify the suspect or say what might have motivated the shooting at YouTube, a video-sharing service owned by Alphabet Inc's Google which employs nearly 2,000 people at the San Bruno, California offices.

The woman approached an outdoor patio and dining courtyard on the campus around lunchtime and began to fire before entering the building, police said.

The San Jose Mercury News, citing a law enforcement source, said that she was targeting her boyfriend due to a domestic dispute.

A U.S. government security official told Reuters there was no known connection to terrorism.

ABC News, citing unnamed law enforcement sources, said the suspect was 35 to 40 years old, and lived in Southern California, with no apparent connection to YouTube.

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," Sherman said in a tweet.

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

The shooting was the latest in a string of mass killings carried out in the United States in recent years. Most recently, the massacre of 17 people at a Florida high school has led to calls for tighter restrictions on gun ownership.

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 of the victims, 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, Dr. Andre Campbell, a trauma surgeon at the hospital, said at a news conference. All three people were victims of gunshot wounds, Campbell said, but none of them had undergone surgery. A fourth person was taken to a local hospital with an ankle injury from fleeing the scene.

YouTube Chief Executive Susan Wojcicki declined to comment to reporters as she left the building.

"It's with great sadness that I tell you - based on the latest information - four people were injured in this horrific act of violence," Google Chief Executive Sundar Pichai said in letter to employees that was posted on Twitter. "I know a lot of you are in shock right now. Over the coming days, we will continue to provide support to help everyone in our Google family heal from this unimaginable tragedy," he added.

In a separate tweet, Pichai said he and Wojcicki were "focused on supporting our employees & the @YouTube community through this difficult time together.”

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

"Our thoughts and prayers are with everybody involved,” 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."

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

Female mass shooters are rare. A recent Washington Post analysis shows only three out of 150 US shootings with more than four victims since 1966 were done by women. In 2015, a husband and wife killed 14 people in San Bernardino, California.