Police in Northern California say they are responding to an incident involving an “active shooter” at YouTube headquarters.
Gunshots were reported at the company’s campus in San Bruno, south of San Francisco.
Several people have been taken to hospital but their condition is unknown.
US media are reporting a female suspect is dead, citing law enforcement sources.
Ambulances are at the scene and authorities have warned the public to stay out of the area.
Zuckerberg San Francisco General Hospital has said it is treating patients from the incident. No details were provided on their condition.
The Stanford Health Care health system has said it is treating four to five people from the shooting, ABC News reports.
One employee at a nearby fast food restaurant told Fox station KTVU he had treated a young woman who suffered a gunshot wound to the leg. He said he had fashioned a makeshift tourniquet from a bungee cord as they waited for first responders.
Some of the injured were reportedly hurt as they fled the building.
Some 1,700 people work at the YouTube HQ. Google said it was “coordinating with authorities” and would provide information as it becomes available.
Images broadcast on local TV stations showed employees leaving with their hands raised. Other footage appeared to show evacuees forming a queue before being individually frisked by police.
Video captured by a nearby helicopter showed a glass door shattered in its centre, with glass fragments strewn across the ground outside.
YouTube product manager Todd Sherman tweeted that people fled the building in panic.