Three people were killed as well as the shooter.
San Francisco police said the gunman, who turned the gun on himself after shooting his coworkers, has been identified as 38-year-old San Francisco resident, Jimmy Lam. The next thing he knew, he said, "there's a mob of UPS drivers" running down the street screaming "shooter, shooter".
Police said Lam was arrested for DUI in 2010 and again in 2013. ABC7 News team coverage of UPS shooting in SF Police sources tell ABC7 News the gunman had a history of mental problems and that he was estranged from his wife. Police also declined to immediately release information about the victims.
At least some of the victims were transported to Zuckerberg San Francisco General Hospital where a spokesman for the hospital, Brent Andrew, said they were treated for their injuries. Police recovered two firearms, one of them a semi-automatic handgun.
Steve Gaut, a UPS spokesman said the shooter was a driver and in uniform. "It's insane." Police responding to the facility in the Potrero Hill neighborhood, about 2 miles from downtown San Francisco, found wounded victims and pulled them to safety.
Video shows UPS employees leaving the area as police swarm UPS facility in San Francisco amid shooting.
UPS employees cross the street shortly after a shooting at the company's facility in San Francisco.More news: Burling, Team New Zealand win Race 7 in America's Cup
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About four hours after the shooting, police were examining and towing away what was believed to be the suspect's vehicle, a BMW sedan, parked near the building.
UPS spokesman Steve Gaut released a statement saying that six employees were involved in a "shooting incident" but that the company can not provide any identifications until the police investigation is complete. They have interviewed all witnesses and are now interviewing victim's families in a search for clues.
"This was a frightful scene", assistant police chief Toney Chaplin said. The four-story building, which covers an entire city block, is visible along Highway 101 as drivers go into downtown San Francisco.
The police declared a shelter-in-place order for the area in the immediate aftermath but lifted the order at around 11:30 a.m.
The San Francisco incident came three years after a UPS employee shot dead two of his supervisors before killing himself at a UPS distribution centre in Birmingham, Alabama.
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In a statement, UPS said that local law enforcement had control of the facility and that the company was cooperating with the investigation.