The man who has been identified by witnesses, fellow UPS employees, and now by police as the sole suspect in Wednesday morning's mass shooting on Potrero Hill, Jimmy Lam, is being described by coworkers and neighbors as a "loner" who never exhibited much of a temper. The 38-year-old Lam had worked for UPS for 18 years, since 1999, had filed a grievance with his union about excessive overtime in March, and at least one other UPS employee told ABC 7 on Wednesday that he "had it out for a manager," though Lam's exact motives in the killings of three fellow employees remain under investigation.

Lam's three victims have been identified as 56-year-old Wayne Chan of San Francisco, 46-year-old Michael Lefiti of Hercules, and 50-year-old Benson Louie of San Francisco. Speaking to CBS 5, Joseph Cilia, secretary treasurer of Teamsters Local 2785, described the scene of the shooting as a regular morning meeting called a pre-communication meeting or PCM, attended by 30 to 60 drivers. "When the meeting started [Lam] walked up to Wayne, then Benson and shot them," Cilia tells the station, explaining that Lefiti, known by many as "Big Mike," ran out of the building and Lam chased him and killed him outside.

Two other drivers were hurt by bullets, but both may have only been hit by ricochets, and both have been released from the hospital.

As many as 350 people work at this UPS sorting facility, and dozens ran for cover as the shooting unfolded, some jumping aboard a passing Muni bus and screaming "Go, go, go!" to the driver as they ducked for cover, per the Associated Press.

Cilia tells the AP that Lam didn't seem angry, and that such grievances were fairly common with drivers, complaining of working more than nine and a half hours three times in one week.

"The overtime has increased over the years," Cilia tells NBC Bay Area, explaining that it's typically common at Christmas but that there has been an uptick in grievances overall in recent years — perhaps as more and more consumers rely on Amazon and the like for their shopping.

Cilia further tells CBS 5 that in all the 18 years he'd known Jimmy he'd never even seen him exhibit a temper. "Maybe something bottled up," says Cilia. "Senseless."

Lam had a criminal history involving two DUIs, in 2010 and 2013, but otherwise neighbors tell CBS 5 he "usually kept to himself" and they would sometimes see him smoking out of the window of his Richmond District apartment, where he apparently lived alone. He was reportedly estranged from his wife and was the father of children.

Next-door neighbor Jennifer Plog tells the Chronicle, "I’m a little creeped out. It’s creepy knowing that people next door have the ability to have guns and shoot people they work with. It makes me feel unsafe.”

On Wednesday, the SFPD towed Lam's BMW from where it was parked near the UPS facility, and conducted a search of his apartment, removing a computer and multiple bags of evidence. Per the Chronicle, it remains unclear whether the two guns recovered at the scene of the shooting were purchased legally.

A retired UPS employee tells CBS 5 that all he remembers of Lam was that he occasionally had "friction" with management.

Meanwhile, UPS customers in the Diamond Heights neighborhood of San Francisco began mourning the UPS delivery man they've known for years, who took his breaks in the Safeway parking lot there. Big Mike Lefiti's regular route was in Diamond Heights, and as CBS 5 reports, the Safeway nearly sold out of flowers Wednesday as the community created a makeshift memorial for the man outside the store, describing him as a "gentle giant with a big smile."

Lefiti, a father of five, had just celebrated his seven-year wedding anniversary, and had worked for UPS for 17 years — nearly as long as Lam.

Multiple members of Lefiti's extended family gathered at SF General to mourn their loss on Wednesday. Cousin Mack Toia, who arrived to pick up Lefiti from work Wednesday morning just as the shooting occurred, explains to ABC 7 that they come from a tight-knit San Francisco family.

Potrero Hill resident Mike McDonald was the last person to speak to Lefiti as he bled out on the pavement. He came upon him as the shooting was still in progress, and says that paramedics could not approach because the police were still trying to secure the area. McDonald tells CBS 5, "He didn’t understand why the guy shot him... He knew who he was. He said he was a driver and he said he had no bad blood with him." McDonald says that Lefiti wanted to him to relay a message to his family and kids, "He just wanted to tell them that he loves them and everything like that."

Previously: UPS Shooter Identified As Richmond District Father; Primary Target Was Allegedly Manager