What we know:
‣ Police confirm female shooter was found deceased inside the facility with "self-inflicted" gunshot wounds.
‣ Police have identified the shooter as 39-year-old San Diego resident, Nasim Najafi Aghdam.
‣ Aghdam, an animal activist, was reportedly seeking revenge on the video streaming company, who had allegedly removed and censored her video content in 2017.
‣ Four were transported to hospital, three for gun-related injuries: a 36-year-old man is in critical condition, a 32-year-old woman is in serious condition and a 27-year-old woman is in stable condition in hospital.
‣ Police responded to alerts of an active shooter at 12.46pm local time on Tuesday. They arrived at the scene two minutes later.
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President Trump has been briefed and has tweeted his thoughts and prayers to the victims.
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Authorities were called to YouTube headquarters in San Bruno, San Francisco, following reports of an "active shooting" inside the facility.
International media reported multiple gunshots were heard just before 1pm local time and employees were being evacuated from the building.
San Bruno Police Chief Ed Barberini said one female was found deceased with "self-inflicted" gunshot wounds during a Police search of the building. It is believed she was the shooter.
A spokesperson for San Francisco General Hospital told CBS it had received three patients including a 36-year-old man in critical condition, a 32-year-old woman in serious condition and a 27-year-old woman in stable condition.