A San Francisco UPS driver who killed three colleagues in a shooting rampage at a company warehouse was armed with two stolen guns and appeared to target his victims, police said Friday.

Police Commander Greg McEachern told a news conference that investigators have not yet determined Jimmy Lam's motive, but "the shooting appeared purposeful and targeted."

Lam, 38, shot and killed three fellow drivers and wounded two others before killing himself in front of police in the June 14 shooting. An 18-year veteran of UPS, Lam opened fire at a morning meeting of UPS drivers before the drivers went out on deliveries.

Lam began by shooting Benson Louie, 50, in the meeting before turning the gun on Wayne Chan, 56, McEachern said. He then walked outside and confronted Mike Lefiti, 46, and fatally shot him multiple times.

Lam then walked back inside the warehouse, where police found him after a two-minute search. Lam walked by several other employees he could have shot but didn't, McEachern said.

When police confronted Lam, he had the gun pointed to his head and police ordered him to drop the weapon. Instead, he shot himself, McEachern said.

Lam didn't say anything during the rampage, McEachern said.

Lam was armed with two stolen weapons. One was an assault pistol that is illegal in California and was stolen in Utah, which he used in the killings, McEachern said. Lam also had a semi-automatic handgun stolen from Napa County, California.

It was not known who stole the weapons, McEachern said.

A San Francisco Police Department official told The Associated Press last week that Lam appears to have felt disrespected by co-workers, but it's not clear what role that feeling played in the shooting.

"We are going to work painstakingly to try to get a motive," McEachern said.

Friends and colleagues have said that Lam struggled with personal issues and was depressed a few years ago. Lam had shown improvement, but he started to look troubled a few weeks ago, they said. That was around the time Lam filed a grievance in March claiming he was working excessive overtime.

Investigators seized multiple phones, computers and a journal that forensic experts are attempting to gain access to and read, McEachern said.

Lam shot his victims with a MAC 10 made by MasterPiece Arms, an automatic pistol that is illegal in California. The pistol was also equipped with a special magazine that held 30 bullets instead of 10, gun gear that is also illegal in California, McEachern said.

Lam fired a total of 20 shots and didn't use the other gun recovered at the scene. Police found a box of bullets in Lam's backpack.

Investigators have interviewed more than 100 people and are re-interviewing key witnesses, McEachern said.