A driver armed with a handgun opened fire at a United Parcel Service Inc package-sorting center in San Francisco on Wednesday, killing three people before fatally shooting himself as officers closed in.

Wednesday’s shooting prompted a massive police response in a neighbourhood near the California city’s downtown area.

San Francisco Assistant Police Chief Toney Chaplin told journalists the employee killed three people and wounded two others, before turning the gun on himself.

The man opened fire while the workers were gathered for their daily morning meeting before they were due to head out on their delivery routes, UPS spokesman Steve Gaut told The Associated Press.

The victims, like the gunman, were also company drivers, he confirmed.

Police offered no explanation as to a possible motive for the shooting, but Chaplin told a news conference the shooting was not an act of terrorism.

Police recovered two firearms from the UPS facility, including the murder weapon, which they described as an “assault pistol.”

The UPS facility, which employs about 350 workers in the city’s Potrero Hill area, was initially placed under a security lockdown as a precaution.

The gun violence erupted just hours after a different gunman opened fire on Republican lawmakers at a congressional baseball practice, wounding US Rep. Steve Scalise of Louisiana and several others.