Hydro One, one of the largest electric utilities in Canada, has grounded all eight of its helicopters after four utility workers died in a crash last week. Helicopters will remain on the ground until a thorough review of safety protocols and equipment is complete, reports 570 News.

The four workers were completing work on a transmission tower at the time of the crash, according to a statement from Hydro One, including one helicopter pilot and three power line technicians. As of Dec. 15, the cause of the crash was still unknown, but Hydro One is working with support from the Transportation Safety Board of Canada to identify the cause.