Abstract: At 3:15 p.m. on May 19, 2020, nine employees were at work. This investigation covered three firms, all in the same city. One firm constructed power lines, communication lines, and related structures. It had inspection number 1475852. Another firm was a temporary staffing employment agency. It had inspection number 1476196. The third firm operated a storage terminal. It had inspection number 1475880. This firm was operating a tank terminal for oils, gases, and chemicals. It appears that the temporary staffing employment agency had sent one or more of its employees to work for the construction firm at the storage tank site operated by the storage terminal. One or more employees had been instructed to weld in the presence of an explosive atmosphere while modifying an improperly prepared storage tank, storage tank 106-37. Pads were being welded onto the tank roof. The tank had contained low-sulfur vacuum gas oil. Flammable vapors were still present. The vapors ignited. An explosion occurred inside the tank followed by a fire. One employee fell 40 feet (12.2 meters). He suffered a fractured hip or pelvis and a fractured arm when he struck the ground. A second employee, working as a helper, suffered burns. Those two employees were hospitalized for two or more days. The other seven employee were either not injured at all or were treated without hospitalization. The narrative did not clarify what each of the nine workers was doing at the time of the incident. The incident was covered by one UPA, number 1593040. The storage terminal received the citations.