SAN FRANCISCO >> A man who fatally shot three fellow UPS drivers appears to have felt disrespected by co-workers, but it’s not clear if that was the motivation for the bloodshed, a police official said today.

The San Francisco Police Department official could not provide details and cautioned it’s one of several possible motives for Wednesday’s shooting. The official spoke on condition of anonymity because the investigation is ongoing and the San Francisco police are not discussing the case publicly.

Jimmy Lam, 38, opened fire at a morning meeting of UPS drivers at a company warehouse in San Francisco before the drivers went out on deliveries. Lam and the victims worked out of the warehouse.

Shaun Vu, a senior UPS driver, has said Lam also struggled with personal issues and was depressed a few years ago. He had shown improvement, but Vu said Lam looked troubled a few weeks ago. That was around the time Lam filed a grievance claiming he was working excessive overtime.

The police official said Lam appears to have targeted the three drivers he fatally shot. It’s not clear, however, that those drivers — Benson Louie, Mike Lefiti, and Wayne Chan — had anything to do with his apparent sense of disrespect.

UPS driver Jevir Gray, 34, rejected the possibility that the victims disrespected Lam, saying they were “really good people.”

“They were my mentors,” he said. “I looked up to them.”

Vu has said Lam seemed to be on friendly terms with the three men he killed.

UPS driver Leopold Parker, who witnessed the shooting, said drivers at the warehouse generally got along and didn’t mind working there. If they did have a problem with colleagues, they would talk to them or ignore them. He also stressed that drivers spent much of their time alone in their trucks, so they had limited interaction with their colleagues.

Lam sometimes complained about the workload, but Parker said he never suspected Lam would turn violent.

Parker, 53, was standing by his truck at the morning meeting when Lam walked up and shot Louie in the head, he said. Louie was a few feet in front of Parker. Lam glanced at Parker.

“I heard a pop. I saw a flash. I saw Benson drop in front of me,” he said. “He glanced at me, so I thought I was next cause I was right next to Benson.”

But Lam went the opposite way. Parker jumped into the cab of his truck and said he saw a couple of bodies on the ground when he looked over. He ran to the roof of the building after he saw Lam running toward an exit.

Joe Cilia, an official with the union that represents UPS workers in San Francisco, has said witnesses told him that after shooting Louie, Lam shot Chan in the back and then walked up and “finished him.”

Lefiti was fleeing from the building when Lam went to a street and shot him, he said, citing witnesses.

The violence ended when Lam turned the gun on himself and took his own life as workers ran from the packing facility and police closed in, police have said.