The San Bruno Police Department Chief Ed Barberini said police were called to YouTube headquarters at 12:46 p.m. local time and were on campus within two minutes.
Several media reports have indicated the suspected shooter is female.
Twitter user @Lavrusik, who identifies as a YouTube employee on his account, said he was evacuated from the building after hearing shots and barricading himself in a room with coworkers.
Zuckerberg General Hospital told NBC News that it had received three patients and was expecting more, while Stanford Medical Center said it was expecting four or five patients. Barberini said officers found a wounded victim outside of the building.
Google, YouTube's parent company, said in a statement that they are "coordinating with authorities and will provide official information here from Google and YouTube as it becomes available".
There are multiple gunshot victims being treated at various hospitals in the Bay Area. Spokesman Brent Andrew says a 36-year-old man was in critical condition, a 32-year-old woman was in serious condition and a 27-year-old woman was in fair condition.More news: President's attorney floated idea of Flynn, Manafort pardon
YouTube employees began tweeting about the presence of an active shooter at the company's Cherry Avenue headquarters at around 1 p.m. PST.
Police in San Bruno warned people in a Twitter message to stay away from the address where YouTube, owned by Alphabet Inc's Google (GOOGL.O), is based. Police officers could be seen patting down employees evacuating the campus to a nearby parking lot, where they were surrounded by police cars. Heard shots and saw people running while at my desk.
The ATF is also responding to the shooting.
"We are responding to an active shooter".
The White House said President Donald Trump had been briefed and that his administration was monitoring the ongoing situation in San Bruno.