Investigators looking into the derailment Friday night of a super-express train on the JR Nagasaki Line in Isahaya, Nagasaki Prefecture, determined Saturday that it was caused by a 130-kg rock that slipped onto the tracks.
The train collided with the rock and the lead carriage rolled over, injuring 33 people.
The Nagasaki Prefectural Police and the Land, Infrastructure and Transport Ministry agreed on the cause of the accident after conducting an investigation earlier in the day. The accident occurred at about 9:45 p.m. on a stretch between Hizen Nagata and Oe stations in Isahaya's Koten district.
The police plan to continue the probe and may charge the train company with professional negligence due to lax security management.