At least three people were injured before the shooter reportedly turned the gun on herself in a domestic-related incident.
"This is an active event and an active investigation and numerous law enforcement personnel from numerous different local agencies are still conducting a search of the business just to make sure that it's clear," San Bruno police chief Ed Barberini told reporters, according to the ABC.
Police responded to the scene after receiving multiple emergency services from terrified employees inside the Silicon Valley building, reports SMH.
The incident is being treated as a case of domestic or workplace violence rather than an act of terrorism, the LA Times reports. It is thought the shooter's partner, who was injured in the shooting, works at YouTube.
US President Donald Trump has responded to the incident with a statement: "Was just briefed on the shooting at YouTube’s HQ in San Bruno, California. Our thoughts and prayers are with everybody involved. Thank you to our phenomenal Law Enforcement Officers and First Responders that are currently on the scene."