The shooter has been described by US media only as a white female wearing a headscarf. The office building has been put on lockdown as police deal with the situation. Police have warned via Twitter to stay away from the area.
Many workers, and those in buildings adjacent, have taken to social media to share updates.
At a police press conference, San Mateo County Sheriff's Department revealed the shooter was deceased, but did not comment on the widespread media reports about the shooter's gender and reported relationship with one of the victims.
However, two sources told ABC7 News the shooter was a white, adult female.
'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.
'Two additional victims were located several minutes later at an adjacent business.'
While no deaths have been confirmed besides the apparent shooter, there have been reports of multiple casualties.
San Francisco General Hospital confirmed an unverified number of patients being admitted from the shooting, though their injuries are not yet known.
Stanford Hospital also confirmed taking in four or five patients injured in the shooting.
More to come
Police have confirmed the female shooter was found deceased at the scene via a self-inflicted gunshot wound.