SAN FRANCISCO — Jimmy Lam had his troubles: a drunk driving conviction, a grievance against his company, a struggle with personal issues and depression a few years ago.

But colleagues and investigators say none of it helps explain why the UPS driver showed up at his San Francisco workplace and gunned down three colleagues, wounding two others.

Lam, an 18-year veteran of UPS, appeared to single out the three slain drivers, but investigators have yet to determine a motive for Wednesday’s attack, an official in the San Francisco Police Department said.

The official was not authorized to discuss the case publicly and spoke on the condition of anonymity.

The violence ended when the 38-year-old Lam turned the gun on himself and took his own life as workers ran from the packing facility and police closed in.

Shaun Vu, a senior UPS driver, said Lam had seemed to be on friendly terms with the three men he killed.

Lam had struggled with depression and had personal troubles a few years ago that involved a dispute with a girlfriend over visitation rights for their young child. Vu said he encouraged Lam to seek counseling.

“I told him, ‘You have a problem or feel bad about yourself or your life, the best thing to do is get professional help,”’ he said.

Lam told Vu that he would speak with management and then took off work for several months. He seemed fine when he returned to work, but Vu noticed a few weeks ago that Lam looked troubled.

Lam also had a history of off-duty driving violations.

He was convicted twice of driving on a suspended license in 2013 and 2014, according to DMV records. His license was suspended in 2014 for negligently operating a vehicle.

Lam also had a run-in with the law in 2010, when he was convicted in San Francisco of driving under the influence and sentenced to three years’ probation.

In March, Lam filed a grievance claiming he was working excessive overtime, said Joseph Cilia, Lam’s friend and an official with the union that represents UPS drivers. Cilia said Lam was upset with managers whom he accused of forcing him to work overtime. But none of the men who were shot were managers, Cilia noted.

Cilia said witnesses told him that Lam walked up to driver Benson Louie and shot him during a meeting. As his co-workers fled, he shot Wayne Chan in the back. Victim Mike Lefiti was fleeing when Lam went to a street and shot him.