Police in Northern California are responding to reports of an "active shooter" at YouTube headquarters in the city of San Bruno.
City Manager Connie Jackson says there have been multiple 911 calls reporting a shooting and that police and fire officials have responded.
San Bruno police warned on Twitter to stay away from the area housing the headquarters of the Google-owned video sharing service near San Francisco.
"We are responding to an active shooter. Please stay away from Cherry Ave & Bay Hill Drive," the police department tweeted.
Google tweeted: "Re: YouTube situation, we are coordinating with authorities and will provide official information here from Google and YouTube as it becomes available."
Police say four people have been injured in the shooting and the extent of injuries are unknown.
One deceased female with self-inflicted wounds was found in the building and investigation is still ongoing.
Northern California's Stanford Hospital says it has received four to five patients from a shooting at YouTube headquarters.
Hospital spokesperson Lisa Kim did not have information on their conditions or their wounds.
Some employees recounted frantic scenes as they fled, with one saying he saw blood on the floor as he escaped.
"We were sitting in a meeting and then we heard people running because it was rumbling the floor. First thought was earthquake," employee Todd Sherman tweeted.
"I looked down and saw blood drips on the floor and stairs. Peeked around for threats and then we headed downstairs and out the front."
TV helicopter video shows officers patting down people grouped outside a building and police vehicles surrounding the area. San Bruno police said they could not provide more information.
YouTube employee Vadim Lavrusik posted on Twitter that he heard gunshots and saw people running. He said he was barricaded in a room with his co-workers before being safely evacuated.
Will Hudson said his friend who works for YouTube texted him about the shooter.
"I think there might be a shooter in my building," read one text. "The fire alarm went off so we started to evacuate and then people (started) running saying there was a shooter."
Hudson said his friend has made it safely back to San Francisco and is in contact with his family. Hudson said he's become accustomed to hearing about such violence but has never been so close to it before.
"It just feels strange. It feels like it could really be anyone. That's really the strangeness of it," he said.
The world's biggest online video website is located on a suburban campus.
Calls and emails to YouTube representatives were not immediately returned Tuesday afternoon.