United Parcel Service (UPS) trucks are rolling again at a warehouse in San Francisco one day after an employee shot and killed three colleagues and wounded two others before killing himself in front of police officers.
Investigators are trying to determine what prompted Jimmy Lam, 38, to open fire during a meeting with co-workers.
Lam, a UPS driver, had filed a grievance claiming that he was working excessive overtime and appeared to specifically go after the three drivers he killed before fatally shooting himself.
He filed the grievance in March complaining of too much overtime and requesting that the package delivery company should relieve him of working extra hours, a union chief said.
During a meeting on Wednesday morning, Lam walked up to driver Benson Louie and shot him. As his co-workers fled the room, Lam shot Wayne Chan in the back, and then walked up to him and "finished him", according to Teamsters Union official Joseph Cilia.
"Mike Lefiti was fleeing from the building when Lam went out onto the street and shot him. It's senseless. I can't think of anything. Why him? Why them? I can't put it together," said Mr Cilia.
Officials confirmed San Francisco residents Louie, 50, and Chan, 56, and 46-year-old Lefiti, of Hercules, were killed in the shooting.
Two other UPS employees were wounded, but Mr Cilia said both were released from the hospital.
The shooting prompted a massive police response in one of the city's industrial areas, about two miles from San Francisco city centre.