SAN FRANCISCO — A gunman killed three co-workers and wounded two others at a sprawling UPS facility here Wednesday before fatally turning the gun on himself, UPS and police said.

Responding officers encountered multiple shooting victims, Assistant Police Chief Toney Chaplin with the San Francisco Police Department, said during a news conference outside the building. One of the police teams located the shooter, who was armed with an assault pistol.

"The suspect put the gun to his head and discharged the weapon,” Chaplin said.

The gunman was given medical treatment at the site but died. He was wearing a UPS uniform but Chaplin said he could not confirm he was a UPS employee. Earlier, UPS said in a statement the shooter was an employee. The wife of one UPS employee described a chaotic scene inside the UPS facility, where the assailant shot several people execution-style.

Police are not releasing information about the victims until families are notified, Chaplin said.

Police sealed off streets in city's Potrero Hill area after the shooting. The facility employs about 350 people, UPS said.

Zuckerberg San Francisco General Hospital, located about a mile from the facility, was put on alert following the shooting.

The large UPS processing facility is about two miles south of downtown. The four-story building, which covers an entire city block, is clearly visible along Highway 101 as drivers go into downtown San Francisco.

KPIX-TV reported the shooting happened during the daily 9 a.m. drivers' meeting. Auto shop owner Robert Kim told KPIX he heard five to eight rapid gunshots, then saw "a mob of UPS drivers” running down the street screaming “Shooter, shooter!”

A long column of two rows of UPS employees were escorted out of the building by armed police officers, KPIX reported.

The shooting took place in a different part of the building from the area where customers pick up and drop off packages.

UPS said in a statement that the company was cooperating with the investigation.

“The company is saddened and deeply concerned about affected employees, family members and the community we share. Our thoughts and prayers are with all those touched by this incident,” the statement read.