UPS workers react at the scene where a gunman shot and killed multiple people including himself at a UPS facility in San Francisco, California on June 14, 2017. [Photo/VCG]

SAN FRANCISCO - Six people were shot Wednesday morning at a UPS facility in San Francisco and four were killed, including the shooter.

Assistant Chief Toney Chaplin of San Francisco Police Department (SFPD) told a press briefing that officers at the scene confronted the shooter, who then turned an assault pistol to his own head and discharged the weapon.

Two guns were recovered from the facility, said Chaplin.

The gunman was identified by authorities as San Francisco resident Jimmy Lam.

Unconfirmed media reports based on a witness account said the gunman shot victims in an "execution-style."

The incident took place before 9:00 a.m. local time at the sprawling UPS, or United Parcel Service, warehouse and customer service center in the Portero Hill neighborhood of the city on the US West Coast.

While witnesses said the shooter was wearing a UPS uniform, and the package delivery company said earlier that the shooter and those injured are its employees, Assistant Chief Chaplin declined to identify those killed and injured.

Police treated the facility as a crime scene and put it on lockdown.

"At this point," Chaplin noted, "we do not believe this incident is related to terrorism."

Local media quoted witnesses within the warehouse at the time of the incident as saying that the gunman went in through the front entrance and started shooting, without saying anything -- once at the back of a victim and then at the heads of some others.

Employees of UPS were evacuated from the facility as officers tried to search for additional victims and witnesses.

For a moment, appealing that "don't put officers at risk," SFPD posted a message on its Twitter social media network account calling for media to "stop showing video and photos of (police) Tactical Units search the roof and other parts of the scene."

There have been no words on possible motives of the shooter or what led to the workplace shooting.

UPS said in a statement that pending the police investigation, it "cannot provide information as to the identity of persons involved at this time" and that regarding those injured, it is "unsure of their status at this time."

In an unrelated event hours earlier, a gunman opened fire at a congressional baseball practice field in Alexandria, Virginia, injuring five people, including US House Republican Whip Steve Scalise and two police offers.

The shooter, identified as James Hodgkinson III, a 66-year-old white male from Illinois, was wounded in the gun battle with police and later pronounced dead.