SAN FRANCISCO -- YouTube employees came running out of buildings on a sprawling tech campus in San Bruno, California, with their hands in the air and on their heads amid reports of an active shooter Tuesday. One eyewitness saw the scene from across the street at a fast-food restaurant.

"I seen a girl running out and she was shot in the leg," one witness told CBS News. "And then the other people that were behind me, they opened the door and pulled her in, then grabbed a bunch of napkins."

A female shooter entered a building and shot three people, according to a law enforcement source. Police and federal agents quickly began surrounding the building.

"We have one subject who is deceased inside the building with a self-inflicted wound. At this time we believe to be shooter but following up on that," San Bruno Police Chief Ed Barberini said.

An active shooter was reported at YouTube's headquarters Tue., April 3, 2018, in San Francisco, Calif. CBS News

YouTube employees were being patted down by police before leaving. YouTube project manager Todd Sherman wrote on Twitter he "saw blood drips on the floor and stairs":

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. — Todd Sherman (@tdd) April 3, 2018

Three victims were taken to San Francisco General Hospital. The video sharing company has 1,700 employees and is one of the most prestigious places to work in Silicon Valley. Owned by Google, YouTube already employed armed security guards around the campus before Tuesday's shooting.