BISMARCK, N.D. (AP) — Federal investigators believe a broken wheel caused a May 2015 oil train derailment that prompted the evacuation of Heimdal (HYM'-dahl).

The National Transportation Safety Board says investigators found a mark on the track indicating a broken wheel that caused six cars on the BNSF Railway train to leave the tracks.

They also found pieces of a broken wheel at the scene, and say workers had fixed another defective wheel on the train two days before the derailment.

Five derailed cars breached and spilled nearly 100,000 gallons of oil, fueling a massive fire. No one was hurt, but about 30 people had to evacuate.

The train had three locomotives and 109 cars, 107 of them loaded with crude. The broken wheel was on the 81st car. BNSF estimated damage at $5 million.