Investigators raided the home of a United Parcel Service driver who opened fire during a meeting with co-workers at a distribution center in San Francisco, killing three people before fatally shooting himself.
Police were seen carrying dozens of bags out of Jimmy Lam's apartment in the city's Inner Richmond neighborhood Wednesday afternoon in the hopes of determining what prompted the 38-year-old to shoot three UPS employees.
Lam filed a grievance in March complaining of too much overtime and requesting that the package delivery company relieve him of working extra hours going forward, said Joseph Cilia, an official with a Teamsters Union local that represents UPS workers in San Francisco.
Cilia said it was a routine filing for drivers when they worked 9 1/2 hours more than three times a week.
"The overtime has increased over the years," Cilia said. "So there are quite a few drivers where overtime is expected during Christmas and seems to be more frequent throughout the year than it used to be in past history."
During a Wednesday morning meeting, Lam walked up to driver Benson Louie and shot him. As his co-workers frantically fled the room, he shot Wayne Chan in the back, and then walked up to him and "finished him." Mike Lefiti was fleeing from the building when Lam went out onto the street and shot him, Cilia said witnesses told him.
"It's senseless. I don't understand why," said Cilia. "He always greeted me when I saw him. Nothing out of the ordinary."
Authorities have been digging into Lam's past, confiscating a computer tower among the items they hauled out of the duplex as they search for a motive. Sources told NBC Bay Area that Lam may have been upset with his boss.
At the scene, police seized two guns, including a Mac 10-style machine pistol, and a backpack filled with ammunition and three magazines, sources said. NBC Bay Area has also learned that at least one of the guns found at the scene was unregistered.
It was not clear if any weapons or ammunition were recovered from Lam's home.
Neighbors said they’d seen him around in his uniform, but he usually kept to himself, sometimes smoking out his duplex window.
Lam's only previous brushes with the law involved driving under the influence. He was convicted of DUIs in 2010 and again in 2013, records show.
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.
The Associated Press contributed to this report.