SAN FRANCISCO - Police said on Tuesday they were responding to an "active shooter" at YouTube's headquarters near San Francisco, as witnesses described seeing blood on the floor and stairs and casualties were reported.

Authorities say a woman believed to be the shooter is dead of a self-inflicted gunshot and four others have been wounded.

San Bruno Police Chief Ed Barberini told reporters that the victims have gunshot wounds and were taken to hospitals.

A spokesman for San Francisco General Hospital says it received three patients. Spokesman Brent Andrew says a 36-year-old man is in critical condition, a 32-year-old woman is in serious condition and a 27-year-old woman is in fair condition.

He says the hospital is expecting more patients but doesn't know their conditions.

Police in San Bruno warned people in a Twitter message to stay away from the address where YouTube, owned by Alphabet Inc's Google, is based.

"We are responding to an active shooter. Please stay away from Cherry Ave & Bay Hill Drive," San Bruno police said on Twitter. The local CBS affiliate reported that casualties were taken to a San Francisco hospital.

A YouTube product manager, Todd Sherman, described on his Twitter feed hearing people running, first thinking it was an earthquake before he was told that a person had a gun.

"At that point every new person I saw was a potential shooter. Someone else said that the person shot out the back doors and then shot themselves," Sherman said on Twitter.

"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.

A police dispatcher said by telephone that it was an "active situation," declining to elaborate.

Google told NBC news in a written statement that it was coordinating with local authorities.

"Customers said they heard what could have been gunshots when they were on their way here," Natalie Mangiante, an employee at Big Mouth Burgers located near the YouTube building, said by phone. Mangiante said she did not see or notice anything.

Local television images showed YouTube employees walking out of the building with their hands raised.

Last month, YouTube announced it would ban content promoting the sale of guns and gun accessories as well as videos that teach how to make guns.

Reuters and AP