SAN FRANCISCO — UPS trucks are rolling again at a warehouse in San Francisco where an employee shot and killed three colleagues and wounded two others.

Workers returned to the facility Thursday, a day after the shooting led police to evacuate the warehouse.

Police say the three UPS drivers were killed by 38-year-old Jimmy Lam, who fatally shot himself in the head in front of officers.

A motive in the killings has not been determined, but a union official has said Lam filed a grievance over what he considered excessive overtime.

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