UPDATE: A gunman in a UPS uniform killed three people and wounded two others before turning his weapon on himself as police approached at a company facility in San Francisco early Wednesday, authorities said.

The San Francisco Police Department said three people were killed and the suspect was dead from a self-inflicted gunshot wound during a press conference. Two others were wounded.

Assistant Chief Toney Chaplin said at the press conference that officers responded to a call of several people shot at the UPS facility at around 8:55 a.m. local time (11:55 a.m. ET).

Officers determined the shooter was still in the building and when they encountered the armed suspect he "put the gun to his head and discharged the weapon," Chaplin said.

Chaplin said they believed that there was only one suspect in the shooting and that the incident did not appear to be related to terrorism.

He added that the suspect was "dressed in a UPS uniform," but that investigators had not yet determined or confirmed whether he was a current employee. Investigators were still working to determine a motive.

There were "dozens and dozens of witnesses" to the shooting, Chaplin said.

"Our investigators are going through, painstakingly interviewing with everyone that was in the building when this tragedy occurred," he said.

UPS said in a statement that there was a "shooting incident involving four employees" within the company's facility in the neighborhood of Potrero Hill.

The company said it could not provide the identity of the people involved pending the police investigation.

"We understand that there are potentially multiple deaths, although some individuals were transported to the hospital and we are unsure of their status at this time," UPS said in the statement.

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

The Bureau of Alcohol, Tobacco, Firearms and Explosives said on Twitter that they were responding to the shooting incident and providing report to San Francisco police.

PREVIOUS STORY: At least five people were shot at a sprawling UPS warehouse and customer service center in San Francisco, authorities confirmed to NBC News.

The shooting early Wednesday led to a massive police response and a shelter-in-place warning for the Portero Hill neighborhood and surrounding area

A UPS employee in his uniform opened fire on employees.

San Francisco police responded to the UPS warehouse on 17th Street and Potrero Avenue, according to a tweet from the San Francisco Police Department.

Authorities had earlier said at least two people were being treated for injuries in connection to the shooting. An official later confirmed that at least five people were shot and that two were "medical examiner cases." It was not immediately clear if they were deceased.

Police activity was first reported near 17th Street and Potrero Avenue, according to a tweet from the San Francisco Police Department at 9:06 a.m.

"#SFPD is at the scene of a shooting that occurred near 17th & Vermont. Please avoid the area, expect street closures and traffic delays #SF," the tweet from SFPD said. Police are asking people in the area to shelter in place. 

According to Francis Zimora from the San Francisco Department of Emergency Management, multiple law enforcement agencies are responding to a scene with multiple casualties. Zimora could not confirm on the status of the victims. 'The most important thing right now is to avoid the area," Zimora said.

Sources said multiple ambulances rolling to the scene.

According to video from NBC Bay Area's SkyRanger, UPS employees could be seen coming out of the facility in a single file, surrounded by police.

A witness tells NBC News she was driving up Bryant Street with her husband when she saw dozens of UPS employees running in the opposite direction.

Sarah Meier-Heredia said she at first thought it was some sort of “group activity.” But the closer she got to the UPS facility, she saw employees running “haphazardly” in all different directions with a look of “panic on their faces.” “When we were immediately outside the facility entrance, I saw a tall man that looked to be directing people out and away from the building,” she said. “As the final people came running out, I heard multiple gun shots. At first I thought eight or nine, but maybe more like five or six. Rapid succession, bam bam bam bam bam bam.” She said she ducked in the car while her husband drove off quickly.