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A shooter on Tuesday wounded at least three people at YouTube’s headquarters in San Bruno, California, according to police.

“Active shooter at YouTube HQ,” Vadim Lavrusik, a YouTube employee, tweeted. “Heard shots and saw people running while at my desk. Now barricaded inside a room with coworkers.”

San Bruno police confirmed at least one person — a woman whom they believe to be the shooter — was dead of a possible self-inflicted gunshot wound. At least four people were injured, three of whom were shot, according to San Bruno Police Chief Ed Barberini.

This story is developing, and the investigation has just started. Details could change with more information. Here’s what we know — and don’t.

What we know

The shooting at the YouTube headquarters was first reported at 12:46 pm PT, and police arrived on the scene in two minutes. People were fleeing the building when law enforcement arrived; police encountered one victim with an apparent gunshot wound near the front of the building, according to police chief Ed Barberini.

One woman — believed to be the shooter — is dead due to what police believe to be a self-inflicted gunshot wound, Barberini said at a press conference.

The alleged shooter, identified as 39-year-old Nasim Aghdam, was reportedly upset about YouTube’s policies and practices, according to police.

At least four people have been transported to hospitals, three of whom suffered gun-related injuries, Barberini said.

Zuckerberg San Francisco General Hospital received three gunshot victims, hospital spokesperson Brent Andrew said at a press conference. There are two female victims; one, age 32, is in serious condition and the other, age 27, is in fair condition. One male victim, age 36, is in critical condition. The hospital is not expecting more patients, the spokesperson said.

A fourth person was hospitalized with an ankle injury; the person was possibly hurt as he or she was trying to flee, according to police.

The shooting occurred in an open courtyard-like area inside the complex, Barberini said.

Police say a handgun was used in the attack.

Employees at YouTube reported hearing shots and seeing people fleeing.

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) 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) April 3, 2018

Police told the public to stay out of the area.

Some employees were safely evacuated between 15 and 20 minutes later.

I got evacuated outside with my hands up. I’m with other people. I don’t think the shooter’s been found that I know of. I saw blood drops on the stairs I walk up everusay. I’m shaking. This is surreal. I hope my colleagues are okay. — Lil | Milktea (@_lilchen) April 3, 2018

There were multiple 911 calls related to the incident, the city manager of San Bruno told the Associated Press.

According to Recode: “The YouTube building is in a busy area of San Bruno, which is near the San Francisco airport. It is run in a separate location from the campus of Google in Mountain View, which is further south. The huge video platform is a unit of the search giant, which, in turn, is part of Alphabet.” The building’s main lobby is relatively accessible from the street, although it has security.

Google CEO Sundar Pichai released a statement to employees about the shooting, saying the company was “very actively monitoring and working with local authorities and hospitals.”

Trump tweeted about the shooting, offering “thoughts and prayers” and thanking police for their response to the scene:

Was just briefed on the shooting at YouTube’s HQ in San Bruno, California. Our thoughts and prayers are with everybody involved. Thank you to our phenomenal Law Enforcement Officers and First Responders that are currently on the scene. — Donald J. Trump (@realDonaldTrump) April 3, 2018

What we don’t know