Four people have been injured by a female shooter who killed herself after going on a gun rampage at YouTube's California HQ.
According to witnesses cited by Fox News, the woman shot her boyfriend then herself. She was dressed in a headscarf and was an adult white female.
Four people were taken to hospitals in San Francisco and their conditions are not yet known.
The first reports came at 12.46pm when dozens of employees called police. Upon arrival, police found the woman dead inside the campus.
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Employees are being evacuated from YouTube's HQ in San Bruno, California, amid reports of an active shooter
An injured woman stands outside YouTube's headquarters in San Bruno, California. She was bleeding from the leg and had a scratch on her face but is not thought to be one of those shot
The woman was alert and speaking to police outside the HQ. Four others remain in unknown conditions
Some of the victims were taken to a Carl's Jr restaurant which is next to the property immediately after being injured. It is not clear how serious their injuries are.
At a press conference at around 3.30pm local time, San Mateo County Sheriff's Department confirmed the shooter was dead.
'At 12.46pm this afternoon, we received numerous 911 calls regarding gun shots at YouTube's campus.
'Police arrived at 12.48pm and immediately began a search for a possible shooter or shooters.
'Upon arrival, police encountered numerous employees fleeing. It was very chaotic.
'Officers encountered one gunshot victim... several minutes later while searching the building officers located a second gunshot wound victim that may have been self-inflicted.
A YouTube employee cries on the phone after escaping the shooting on Tuesday afternoon
'Two additional victims were located several minutes later at an adjacent business,' a spokesman said.
The adjacent business was a Carl's Jr on the same plaza where two women fled after being shot.
Disturbing live accounts were given by YouTube employees who took to Twitter as the shooting unfolded.
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In a flurry of tweets, project manager Todd Sherman said: 'We were sitting in a meeting and then we heard people running because it was rumbling the floor. First thought was earthquake.
'After existing the room we still didn’t know what was going on but more people were running. Seemed serious and not like a drill.
'We headed towards the exit and then saw more people and someone said that there was a person with a gun. S**t.
'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.
'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,' he said.
Authorities are now going room to room to find the shooter who appears to still be at large. Photographs show police frisking employees on their way out for weapons.
In an initial statement, Google, which owns YouTube, said it was cooperating with authorities.
A SWAT team is picturing entering YouTube HQ in San Bruno, California, on Tuesday after reports of shots being fired inside
YouTube employees leave the scene on Tuesday after a woman shot four people then herself
'We are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available,' they said.
A worker from Carl's Jr. next door to the campus said one woman was shot in the parking lot and was brought into the restaurant to be cleaned up.
Another witness reported seeing one woman being shot dead in the courtyard outside YouTube.
'It did not stop. There was no mercy,' the witness told Fox News.
One woman who lives next door said she heard rapid fire shots.
'I was in my backyard and I just heard a series of gunshots. It was boom boom boom boom boom and it went on for quite a few seconds.
'I don't know the difference between a handgun but it was rapid fire it was one after another. I heard it get more quiet like he was going away,' she said.
There was an onslaught of emergency vehicles at the scene moments after the shots were fired
Officers are pictured running towards YouTube's San Bruno HQ on Tuesday amid reports of a shooter
Employee Vadim Lavrusik tweeted at around 1pm on Tuesday: 'Active shooter at YouTube HQ. Heard shots and saw people running while at my desk.
'Now barricaded inside a room with coworkers.'
Police at the scene say the situation is a 'very active' scene. San Bruno is around 11 miles south of San Francisco.
Authorities have received 50 separate 911 calls of shots being fired on the premises.
The campus is spread over 200,000 sq ft and is where 1,700 employees work.
There is no specific detail on where the shooting took place, how many people have been shot and whether any have been wounded fatally.
This is YouTube's San Bruno campus where an active shooter was reported on Tuesday at around 1pm