A woman opened fire at YouTube's California headquarters Tuesday afternoon before dying of a gunshot wound, multiple law enforcement sources told NBC News. Multiple injuries were reported.

It wasn't immediately known whether the woman killed herself or was killed by responding security or law enforcement officers.

Little other information was immediately available. 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.

YouTube employees tweeted that they had evacuated the building in San Bruno, south of San Francisco, or were in hiding.

Authorities said they began receiving calls of an "active shooter" about 12:45 p.m. (3:45 p.m. ET). A spokeswoman for the San Mateo County Sheriff's Office confirmed that sheriff's deputies and San Bruno police were responding.

The federal Bureau of Alcohol, Tobacco, Firearms and Explosives also said it was responding.

"Active shooter" is a general police term meaning only that someone may have fired a shot and has not been stopped or taken into custody.

Marco Tartaglia, who works at a Walmart store near the complex on Cherry Street, told NBC Bay Area that he heard about 15 shots "from the direction of the YouTube building, from what sounded like inside."

"We heard the shots first, and that immediately drew us over to the window to see what the heck's going on, and next thing you know, you see people streaming out, just running, dozens of them, just running out the front exit," Tartaglia said.

Google, YouTube's parent company, said only that it was coordinating with authorities.

This is a developing story. Refresh this page for updates.