Two UPS coworkers who witnessed last Thursday's shooting in Potrero Hill, one of whom was shot in the leg during the ordeal, remain confused about what drove 38-year-old Jimmy Lam to kill three men who, by most accounts, he didn't appear to have any particular beef with. UPS driver Leo Parker, who witnessed the shooting first-hand and even exchanged a glance with Lam before diving under a truck to safety, tells NBC Bay Area that the three deceased victims were all close friends at work. "It’s a very shocking thing," Parker tells the station. "Most of it happened right in front of me; most of the images you can’t just get out of your head."

Similarly, driver Alvin Chen tells the Associated Press that he's been deeply affected by witnessing the killings. Chen was one of two drivers who were wounded in the shooting but released from the hospital the same day — and it still seems unclear if Chen was an intended victim, or if he was simply hit by a bullet in the leg because he was standing with two of the victims. Chen says that victims Wayne Chan, 56; Benson Louie, 50; and Mike Lefiti, 46, were all kind and helpful people and it would have been "out of character" for them to have singled anyone out for teasing or ridicule, as has been suggested during the initial police investigation. Chen also tells the AP that he had a friendly conversation with Lam just the day before the shooting, talking about their respective routes, and that nothing seemed out of the ordinary.

The question then remains what made Lam crack and decide to target, very specifically, these three men with whom he had worked for nearly two decades, in the case of two of them. As we learned last week, after shooting Chan and Louie during a routine morning meeting at the UPS distribution center, Lam then chased Lefiti out of the building, where he shot him in the parking lot — and a Good Samaritan who says he comforted Lefiti and spoke to him as he was dying says that Lefiti didn't understand why he was shot and even said something to the effect of "I never had any beef with him."

As we learned over the weekend, Lam had worked for UPS since he was 20 years old, for 18 years, and lived alone in an apartment in the Richmond District. It's been reported that Lam was estranged from a wife and had multiple children, but few details about his personal life have emerged — the AP also reported via some coworkers that Lam was in a dispute with a girlfriend over visitation rights with their young child.

An interfaith prayer vigil was held Monday evening outside the UPS headquarters, and as Mission Local explains it was organized by the Archdiocese of San Francisco Restorative Justice Ministry.

Catholic Archbishop Salvatore J. Cordileone said at the service, "They apparently lived their lives according to the way god calls us to live — injecting much love and care and concern and cheerfulness in the everyday routine situations of life. We give thanks to them and let us learn from them."

As SF Weekly reports, GoFundMe campaigns have been set up for each of the three victims' families, as friends and longtime UPS customers continue to mourn.

Previously: UPS Shooting Suspect Described As 'Loner'; All Three Victims Identified