CHEYENNE, Wyo. (AP) — Brake failure probably caused a freight train collision that killed two crew members in southeastern Wyoming in 2018, according to U.S. accident investigators.

An air flow restriction probably caused the air brake system to fail as the train consisting of three leading locomotives and 105 cars descended a 13-mile (21-kilometer) grade, the National Transportation Safety Board said in a report dated Dec. 29.

The train was going 56 mph (90 kilometers per hour) when it hit the rear of another train on Oct. 4, 2018. The crash killed the colliding train’s engineer, Jason Martinez, and conductor, Benjamin Brozovich, and caused the three engines to derail.

Sixty-five train cars — 57 in the colliding train and eight in the train struck — derailed in the Granite Canyon area 18 miles (29 kilometers) west of Cheyenne.

The wreck caused $3.2 million in damage.

Union Pacific failed to maintain the rail cars in accordance with federal regulations such as regularly performing single rail car air brake tests, according to the NTSB.

The failure contributed to the accident, the federal agency found. Union Pacific already has implemented the NTSB recommendations, railroad spokesman Tim McMahan said Monday.

Communication rules set by Federal Railroad Administration regulations and industry standards also contributed to the crash, the NTSB found.