(Adds latest details from police)
SAN FRANCISCO, June 14 (Reuters) - A UPS driver opened fire with a handgun inside a United Parcel Service Inc delivery center in San Francisco on Wednesday, killing three co-workers before fatally shooting himself as police closed in, authorities and company officials said.
The victims, like the gunman, were all company drivers, said Steve Gaut, head of investor relations at UPS. The attack unfolded as the workers were gathered for their daily morning meeting before they were due to head out on their delivery rounds, Gaut added.
Two other people were taken to an area hospital with gunshot wounds, and another five individuals sustained less serious injuries in a frantic exit from the building, San Francisco police said. Authorities did not immediately identify the gunman or the victims.
Police said the gunman took his own life before officers who swarmed the building ever fired a shot.
Police offered no explanation as to a possible motive for the violence. But Assistant Police Chief Toney Chaplin told a news conference the shooting was not an act of terrorism.
Police said they recovered two firearms from the UPS facility, including the murder weapon, which they described as an "assault pistol."
The UPS facility, a package-sorting and delivery hub that serves the greater San Francisco area and employs about 350 workers in the city's Potrero Hill area, was placed under a security lockdown for six hours.
The gun violence there erupted hours after another, unrelated mass shooting at a charity baseball practice in the suburbs of the nation's capital left a congressman and several others wounded before the assailant was killed by police.
"We are always saddened by the loss of life to gun violence. Any shooting is one shooting too many," San Francisco Mayor Ed Lee said on Twitter.
Gaut said the facilitys employees were dismissed from work once the lockdown was lifted and that most had since left the building. The company is providing trauma and grief counseling.
Video footage from the scene showed a massive police presence near the facility, with workers being escorted outside and embracing one another on the sidewalk. One worker was found by police hiding inside the sprawling building after the shooting, unaware that the violence was over.
"It was a frightful scene," Chaplin said.
The San Francisco bloodshed came three years after a UPS employee shot and killed two of his supervisors before turning the gun on himself at a UPS distribution center in Birmingham, Alabama. That gunman had recently been fired from the facility.
The deadliest mass shooting in modern U.S. history occurred in June 2016 when a gunman claiming allegiance to the Islamic State militant group killed 49 people at the Pulse nightclub in Orlando, Florida.
Gun laws in the United States rank among the most permissive of any developed country, with the right to "keep and bear arms" enshrined in the Constitution's Second Amendment. Efforts to tighten national gun control measures failed after mass shootings at an elementary school in Newtown, Connecticut, in 2012 and the nightclub shooting in Orlando.
(Additional reporting by Alex Dobuzinskis in Los Angeles, Patrick Enright in Seattle and Nick Carey in Detroit; Writing by Jon Herskovitz and Steve Gorman; Editing by Lisa Shumaker)