Sources tell KRON4 that a woman shot her boyfriend.
Alphabet Inc., parent company of YouTube and Google, released a statement on Twitter saying officials are coordinating with police.
The score, which is now in the adjudication phase, could break a 17 year old record!
Lisa Kim, a spokeswoman for Stanford Health Care, said the hospital was receiving between four to five patients from the shooting incident at the YouTube offices. The city has responded with a "massive police and fire response".
YouTube's headquarters houses almost 2,000 employees.
The White House said President Donald Trump was briefed on a shooting at YouTube headquarters and that officials were monitoring it. He also said he barricaded inside a room with co-workers before tweeting he had been evacuated.
"First thought was natural disaster", employee Todd Sherman tweeted.
Another YouTube employee Vadim Lavrusik tweeted: "Active shooter at YouTube HQ".
The San Mateo County Sheriff's Office tells CBS News it is treating the situation as an "active shooter incident" but declined to provide more details.
He said he was safe and on his way home.
Calls and emails to YouTube representatives seeking comment were not immediately returned.
Television images showed people leaving the building in a line, holding their arms in the air for police to inspect as they were leaving the building.
YouTube headquarters is located some 50 kilometres from the main Google campus in Mountain View.
Police Chief Ed Barberini of the San Bruno police has confirmed that a woman who police consider a suspect is dead from an apparent self-inflicted gunshot wound, and that four additional victims with gunshot-related wounds have been transferred to nearby hospitals for treatment.
This is a developing story...