A shooting at YouTube’s California headquarters left at least three people wounded and a female suspect dead of an apparent suicide, police said Tuesday.

The San Bruno police chief, Ed Barberini, said during a news conference that police had responded to 911 calls from the Silicon Valley tech campus and discovered a woman, whom they believed to be the shooter, dead of a self-inflicted gunshot wound. Barberini did not release any information about a possible motive or the suspect’s identity, but said the attack was carried out with a handgun and that there was no further threat to the public.

Four people were transported to local hospitals, police said, including three with gunshot wounds. The three are being treated at San Francisco General hospital, a spokesman confirmed, including a 36-year-old man in critical condition and a 32-year-old woman in serious condition. It was not immediately clear where the fourth victim was being treated and in what condition.

A female shooter is a rarity; an FBI study of 160 “active shooter” incidentsbetween 2000 and 2013 found that only six incidents, or 3.8%, were perpetrated by a female shooter.

News of the shooting initially spread on social media as YouTube employees posted about barricading themselves inside rooms as police and ambulances arrived at the scene.

“Heard shots and saw people running while at my desk. Now barricaded inside a room with coworkers,” Vadim Lavrusik, a YouTube employee, posted to Twitter.

A YouTube employee, Michael Ho, said he was on the phone with his wife in an open-floor-plan area when he saw people running. “At first I wasn’t sure if it was something they were doing for fun,” he said, before noticing looks of panic on people’s faces.

Zach Vorhies, a senior software engineer at YouTube, said that he was at his desk when the fire alarm went off. As he passed through an interior courtyard between the main building and the parking garage he saw a man on the ground with what appeared to be a bullet wound to the stomach. He said he heard a voice he assumed to be the shooter’s shout “come and get me!” and saw police with assault weapons responding.

A project manager, Todd Sherman, said he was sitting in a meeting when he heard people running to leave the building. Upon exiting the room he saw “blood drips on the floor and stairs” and heard people say there was a potential shooter before he managed to escape the building.

“Police cruisers pull up, hopped out with rifles ready and I told them where the situation was as I headed down the street to meet up with a couple team members,” he said.

Aerial footage shot by CBS News showed staff leaving the building with their hands in the air. Offices of other companies nearby were also on lockdown.

By late afternoon, the steady stream of employees leaving YouTube’s hilltop campus in San Bruno, a mostly residential suburb of San Francisco, had ceased. Among the last to leave was the chief executive, Susan Wojcicki, who was flanked by security and did not stop to speak to reporters.

Sundar Pichai, the chief executive of YouTube’s parent company, Google, called the shooting an “unimaginable tragedy” and said the company was working to support the victims and their families, in an email to Google staff. Other tech company executives, including Amazon’s Jeff Bezos, Apple’s Tim Cook and Twitter’s Jack Dorsey, sent messages of support on Twitter, with some, including Uber’s Dara Khosrowshahi, calling for an end to gun violence.

Chris Dale, YouTube’s global head of communications, appeared at an afternoon press conference with police where he expressed “a debt of gratitude” to officers and first responders.

“We are an incredibly tight knit community within YouTube, where it feels like a family,” Dale said. “Today, it feels like the entire community of YouTube and all of the employees were victims of this crime.”

Donald Trump, who was criticized for his slow response to the 14 February mass shooting at a high school in Parkland, Florida, that left 17 students and teachers dead, commented on the shooting on Twitter.

Andre Campbell, a trauma surgeon who is treating three of the gunshot victims at San Francisco General hospital, expressed his frustration at a press conference: “Once again we are confronted with the specter of a mass casualty situation in the county of San Francisco. You would think that after Las Vegas, Parkland, the Pulse nightclub shooting, that we’d see an end to this but we have not.

“Gun violence happens every day in the United States. We’ve got a serious problem we need to address. I don’t have all the answers but at least we need discussion about it nationally.”

The shooting comes during a renewed debate over American gun control laws, following the Parkland shooting, which left 17 students and teachers dead. Hundreds of thousands of Americans demonstrated for stricter gun laws on 24 March in Washington and across the country.

With Republicans in Congress blocking any new gun control legislation, much of the activism after the Parkland shooting has shifted to the private sector, with calls for boycotts and corporate action. When a Fox News host tweeted disparagingly about a Parkland activist not getting into his top colleges, the 17-year-old successfully called for advertisers to boycott her show.

YouTube is one of the companies that introduced new policies after the Parkland shooting. In late March, it quietly debuted restrictions on some gun-related video content, Bloomberg News reported. The National Shooting Sports Foundation, which represents gun manufacturers, called the new policy “worrisome” in a statement to Bloomberg News in late March.

YouTube has also come under scrutiny for the way its platforms have been used after mass shootings to spread conspiracy theories that mass shootings are hoaxesperpetrated to advance gun control and that grieving survivors and family members of shooting victims who appear in the media are “crisis actors”.