“I looked down and saw blood drips on the floor and stairs. Peaked around for threats and then we headed downstairs and out the front,” Sherman said.The shooting was the latest in a string of mass killings carried out in the United States in recent years. Most recently, the massacre of 17 people at a Florida high school has led to calls for tighter restrictions on gun ownership.In a recording of a 911 call posted online by the Los Angeles Times, a dispatcher can be heard saying: “Shooter. Another party said they spotted someone with a gun. Suspect came from the back patio … Again we have a report of a subject with a gun. They heard seven or eight shots being fired.”

Dozens of emergency vehicles quickly converged on the YouTube campus, and police could be seen on televised aerial video systematically frisking several employees leaving the area with their hands raised.One of the victims, a 36-year-old man, was listed in critical condition at San Francisco General Hospital. A 32-year-old woman was listed in serious condition and a 27-year-old woman in fair condition. Authorities did not release names of the victims.

The three patients taken to San Francisco General Hospital were all awake, Dr. Andre Campbell, a trauma surgeon at the hospital, said at a news conference. All three people were victims of gunshot wounds, Campbell said, but none of them had undergone surgery. A fourth person was taken to a local hospital with an ankle injury from fleeing the scene.YouTube Chief Executive Susan Wojcicki declined to comment to reporters as she left the building.

“It’s with great sadness that I tell you – based on the latest information – four people were injured in this horrific act of violence,” Google Chief Executive Sundar Pichai said in letter to employees that was posted on Twitter. “I know a lot of you are in shock right now. Over the coming days, we will continue to provide support to help everyone in our Google family heal from this unimaginable tragedy,” he added.