The trauma from a mass shooting at a United Parcel Service distribution center has made going back to work almost unimaginable for many of the nearly 40 workers who witnessed the June 14 massacre.

As UPS workers were gathered for a regular Wednesday morning exercise meeting at their San Francisco distribution center, a man wearing the delivery company’s trademark brown uniform burst in and opened fire, killing three people and wounding at least two others before fatally shooting himself in the head, police said.