Contra Costa County sheriff's deputies investigate a shooting Thursday in Orinda, Calif. (Ray Chavez/East Bay Times/AP)

Airbnb is banning “party houses” and will work harder to prevent unauthorized events, the company’s chief executive announced Saturday after a Halloween party shooting at one of its rentals last week left five people dead.

Police say people from around the Bay Area arrived in costume at a property in Orinda, Calif., on Thursday for a gathering promoted on social media as an “Airbnb mansion party” — without the permission of the rental host, according to Airbnb. Officers described a chaotic scene of more than 100 frightened partygoers scattered around the property, some of them nursing fresh wounds and others fleeing.

Chief executive Brian Chesky said that in addition to banning “party houses” — rental spaces that consistently disturb neighbors based on standards to be determined, according to company spokesman Ben Breit — Airbnb will create a rapid response team for the issue and start manually screening reservations flagged as “high-risk.”

“We must do better, and we will,” Chesky tweeted. “This is unacceptable.”

The company may remove users who violate its new policies, he added.

According to Breit, the Orinda property had house rules against parties. Airbnb says it has removed the listing from the site and banned the person who rented the home.

Gunfire erupted at the property just before 11 p.m. Thursday in the posh hillside community about 17 miles east of downtown San Francisco. Three victims were pronounced dead at the scene, and another died later at John Muir Medical Center in Walnut Creek, Contra Costa County’s trauma center, according to a statement released Friday by the sheriff’s department. A fifth person died in the hospital late Friday night, the sheriff said in a statement.

All the victims were under 30, according to police, who identified them as Tiyon Farley, 22; Omar Taylor, 24; Ramon Hill Jr., 23; Javin County, 29; and Oshiana Tompkins, 19. All of them were from the Bay Area.

Ambulances rushed injured people to the hospital before midnight, while other victims transported themselves, according to police. At the John Muir Medical Center, one victim was in critical condition, one was in serious condition, and another was treated and released, said Ben Drew, a hospital spokesman.

As of late Friday, police had not identified any suspects nor indicated what led to the shooting. Officers said they found two guns at the house and were trying to determine whether they were used in the shooting or other crimes. Investigators were also examining shell casings, interviewing witnesses and analyzing other evidence, according to police.

Orinda Police Chief David Cook called the investigation “very complex. ”

“This is a wide area with a lot of people, and we’re still trying to wrap our arms around exactly what transpired,” he told reporters Friday.

The deadly outburst rattled neighbors and local officials in Orinda, an affluent community of about 18,000 that hasn’t recorded a homicide in seven years and was recently ranked one of “America’s friendliest towns” by Forbes magazine.

“This is a tragedy of unimaginable gravity,” Orinda Mayor Inga Miller said.

Responding to the shooting on Twitter, California Gov. Gavin Newsom (D) renewed calls for Congress to pass gun-control legislation. “This will barely make the news today. That’s how numb we have become to this,” he said. “Our hearts are aching for the victims and all those affected by this horrific tragedy.”

Airbnb, too, said it was “horrified by this tragedy.”

“[We] are in close communication with Chief David Cook of Orinda Police to offer our support with his investigation into who committed this senseless violence,” Breit, the Airbnb spokesman, told the Mercury News.

Police said they received two noise complaints on the property Halloween night — one at 9:19 p.m., the other at 10:25 p.m. — and dispatched officers about 20 minutes after the second call to see what was going on. Reports of gunfire came in around 10:50 p.m.

The owner of the property, Michael Wang, told the San Francisco Chronicle that he rented the home to a woman who claimed she was organizing a 12-person family reunion.

After receiving noise complaints Thursday from neighbors, Wang checked his home security cameras. He observed well over a dozen people inside and called the police, he told the Chronicle. “They were on the way to go there to stop them, but before we got there, the neighbor already sent us a message saying there was a shooting,” said Wang, 58. “When we arrived there, the police were already there.”

Cellphone footage compiled by local media captured a house crowded with people in costumes dancing and mingling, then running for cover as gunfire rang out.

A neighbor told KRON4 that he had heard “a bunch of gunshots” before he saw and heard “people screaming and fleeing down the hill.”

Footage from the ABC News affiliate showed victims limping from the deadly party and weeping in the street, while paramedics loaded a stretcher into an ambulance. Other witnesses told NBC Bay Area that the party had been attended by many college students.

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