(Adds hospital expecting at least four casualties, shooter possibly still at large, previous dateline SAN FRANCISCO)
SAN BRUNO, Calif., April 3 (Reuters) - Police responded to an "active shooter" at YouTube's headquarters near San Francisco on Tuesday, sending employees scrambling into the street as at least four casualties were taken to local hospitals and witnesses described blood on the floor.
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.
Lisa Kim, a spokeswoman for Stanford Health Care, said the hospital was receiving between four to five patients from the shooting incident at the YouTube offices.
It was not immediately clear if a suspect had been taken into custody. Officers at the scene warned people near the campus to stay back, saying it was still possible someone could shoot through the windows.
Employees who evacuated the YouTube office and nearby buildings huddled under trees in parking lots.
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. (Reporting by Paresh Dave; Additional reporting by Gina Cherelus in New York, Suzannah Gonzales in Chicago and Alex Dobuzinskis and Dan Whitcomb in Los Angeles; Writing by Dan Whitcomb Editing by Peter Cooney and Lisa Shumaker)