UPDATE: Shots fired at YouTube offices in California, casualties reported

The latest on this story:

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.

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.

Police activity at 901 Cherry Ave, please stay out of the area. April 3, 2018

Police activity at 901 Cherry Ave, please stay out of the area. pic.twitter.com/H6iAj0g7ra

— San Bruno Police (@SanBrunoPolice)

We are responding to an active shooter. Please stay away from Cherry Ave & Bay Hill Drive.April 3, 2018

We are responding to an active shooter. Please stay away from Cherry Ave & Bay Hill Drive.

— San Bruno Police (@SanBrunoPolice)

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.

Product manager at YouTube Vadim Lavrusik first tweeted about the incident at 12:57 p.m. on Tuesday.

'Heard shots and saw people running while at my desk,' his tweet stated. 'Now barricaded inside a room with coworkers.'

Active shooter at YouTube HQ. Heard shots and saw people running while at my desk. Now barricaded inside a room with coworkers.April 3, 2018

Active shooter at YouTube HQ. Heard shots and saw people running while at my desk. Now barricaded inside a room with coworkers.

— Vadim Lavrusik (@Lavrusik)

He then tweeted at 1:13 p.m. that an evacuation had occurred.

Another 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.

We were sitting in a meeting and then we heard people running because it was rumbling the floor. First thought was earthquake. April 3, 2018

We were sitting in a meeting and then we heard people running because it was rumbling the floor. First thought was earthquake. https://t.co/gmolQmRXm1

— Todd Sherman (@tdd)

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.

Paresh Dave at Reuters and Gazette staff contributed to this report.