UPS workers gather outside a UPS package delivery warehouse where a shooting took place Wednesday, June 14, 2017, in San Francisco. A UPS spokesman says four people were injured in the shooting at the facility and that the shooter was an employee. (AP Photo/Eric Risberg)

UPS workers gather outside a UPS package delivery warehouse where a shooting took place Wednesday, June 14, 2017, in San Francisco. A UPS spokesman says four people were injured in the shooting at the facility and that the shooter was an employee. (AP Photo/Eric Risberg)

SAN FRANCISCO (AP) — A UPS employee who had recently filed a grievance opened fire Wednesday inside one of the company’s San Francisco packing facilities, killing three co-workers before fatally shooting himself as employees fled frantically into the streets shouting “shooter!,” authorities and witnesses said.

The gunman, Jimmy Lam, filed the grievance in March complaining that he was working excessive overtime, Joseph Cilia, an official with a Teamsters Union local that represents UPS workers in San Francisco, told The Associated Press.

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Still, Cilia said Lam wasn’t angry, and he could not understand why he would open fire on fellow drivers at a morning meeting. Cilia says witnesses told him Lam appeared to specifically go for the drivers who died, chasing at least one of them out of the building.

“I never knew Jimmy to not get along with people,” Cilia said. “Jimmy wasn’t a big complainer.”

Two other UPS employees were wounded, but Cilia said both were released from the hospital.

Amid a barrage of gunfire, some workers sought refuge on the roof of the four-story facility, while others ran outside and pounded on the windows of a public bus, witnesses said.

“They were screaming, ‘Go! Go! Go!’” said Jessica Franklin, 30, who was riding to work when the bus made a regular stop in front of the UPS facility. “As they got on the bus, they were all ducking.”

The shooting that prompted a massive police response in one of the city’s industrial neighborhoods, about two miles from downtown San Francisco, Assistant Police Chief Toney Chaplin told reporters.

UPS spokesman Steve Gaut said the shooter was a company employee. A San Francisco Police Department official identified Lam and said he’s from San Francisco but had no immediate details on his background.

The official spoke on condition of anonymity because the investigation is ongoing.

Officials, UPS employees and witnesses described chaos as shots rang out during a morning meeting before drivers were sent on their delivery routes.

Police have not yet released victims’ names but families and friends identified one of the people killed as 46-year-old Mike Lefiti, a UPS driver.

Lefiti’s cousin, Mack Toia, told KGO-TV (http://abc7ne.ws/2sch61d) he was at the UPS facility waiting to pick up Lefiti when he heard shots. He left his van and saw his cousin sprawled on concrete behind a gate, Toia said.

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“The police officers were right on the scene just like that. I got to touch him, but I couldn’t hug him,” Toia said. “They just pushed me away because they were trying to resuscitate him.”

Toia said he was able to tell Lefiti he loved him.

Co-worker Isaiah Miggins said he saw Lefiti, known as “Big Mike,” as he arrived for work just before 9 a.m., a few minutes before the shooting started. “He was a joyful man. Always happy,” Miggins said.

On social media, heartbroken family members of Lefiti recalled him as a warm-spirited man devoted to his children and family. A photo on his Facebook page shows Lefiti in his brown UPS uniform holding a trophy. He also posted photos of his UPS truck and an award for 15 years of service to the company in 2015.

Neighbor Raymond Deng said he heard up to eight gunshots.

“They were all in rapid succession,” said Deng, a 30-year-old tech worker who lives across the street from the warehouse. “It was like tat, tat, tat, tat, tat, tat, tat.”

Police arrived in minutes.

“This was a frightful scene,” Chaplin said. He said officers found two victims outside and others inside and pulled the wounded to safety as they confronted the gunman, who was armed with an “assault pistol.”

“The suspect put the gun to his head and discharged the weapon,” Chaplin said, adding that police did not fire any shots.

Chaplin said police have not determined a motive and were interviewing families of victims and witnesses to piece together what led the gunman to act.

Mayor Ed Lee condemned the violence and praised authorities for a “very proactive response.”

“It could have been worse,” he said. “Lives were saved today.”

It was not immediately clear how many employees were at the facility, but UPS said the warehouse employs 350 people. The shooter and all the victims were employees, UPS said in a statement.

UPS driver Marvin Calderon told KNTV that he recognized the gunman as a fellow employee but did not know him personally.

“I just started running out like crazy, like I’ve never run before,” Calderon told the TV station.

After the gunfire, auto shop owner Robert Kim said he saw “a mob of UPS drivers” running down the street screaming “shooter, shooter.”

Deng watched from his window in the Potrero Hill section of San Francisco as workers fled the building. He said another group of about 10 people gathered on the roof and held up their hands waving for help.

“I saw police officers go up from the ramp and then storm the buildings,” he said. “It’s crazy.”

The shooting occurred the same day a gunman opened fire on Republican lawmakers at a congressional baseball practice in Virginia, wounding U.S. Rep. Steve Scalise of Louisiana and several others.

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AP writers Janie Har, Sudhin Thanawala and Linda Wang in San Francisco and Mike Balsamo contributed reporting from Los Angeles.