A United Parcel Service driver based out of the company’s San Francisco sorting hub and package delivery center shot and killed three of his fellow workers and wounded two more before turning the weapon on himself late yesterday in a murder/suicide incident that news reports indicated involved overtime grievance issues.

The shooter – identified by law enforcement officials as Jimmy Lam, 38 – reportedly filed a grievance back in March complaining of too much overtime and requesting that the package delivery company relieve him of working extra hours going forward.

In a statement, UPS did not confirm any of the speculation surrounding the reason for the shooting.

“The company is saddened and deeply concerned about affected employees, family members and the community we share,” the company said in its statement. “Our thoughts and prayers are with all those touched by this incident. To assist our employees during this time, UPS has made professional counseling available.”