Using a crane the mast was lifted and attached to the rotary beam support and side legs. It was then lowered and pinned to the set back assembly. The assistant driller decided to vary from previous lift procedures and had the assembly lifted so that they could gain access to bolts that were in the toolbox located in the off driller side of the substructure base. While lifting the assembled section, the chains broke. The Assistant Driller and Floorman were working under the load and the second Floorman was knelling on top of the beam with his fall protection on. Two Floormen sustained injuries and were treated in the hospital, but the Assistant driller was critically injured to head and died on the way to the hospital.