An investigation has found that the improper removal of preventive equipment caused a bridge to collapse in February at a construction site for an expressway linking the central administrative city of Sejong with Pocheon, Gyeonggi Province.

According to the results announced by the transport ministry and the investigation committee on Tuesday, the removal of screw jacks and wires installed to prevent girders from falling was the main cause of the bridge collapse, which killed four workers.

While the screw jacks should have been removed only after the girders were securely in place, the panel concluded they were taken out early for convenience, adding that in structural analyses, the girders did not fall when the screw jacks were left in.

The investigators said another cause of the accident was the improper placement of launcher devices, which are used to lift and position bridge girders, in violation of safety regulations.

The operator of the launcher on the day of the accident was found not to be the same person listed in the construction plan, a sign of insufficient on-site management and supervision, according to the investigators.

The panel also said there is a need to inspect the site more closely to determine whether the structures that did not collapse should be repaired or taken down and built from scratch.