WASHINGTON (AP) — A coal train derailment that killed two Maryland college students was caused by a broken rail on a section of track being monitored by a railroad because of previous problems, the National Transportation Safety Board said Thursday.

The NTSB’s conclusion was part of a 19-page report on its investigation into the 2012 derailment in Ellicott City. The derailment of the CSX train on Aug. 20, 2012 killed Rose Mayr and Elizabeth Nass, both 19, who were spending a summer night together before heading back to college. They were sitting on a railroad bridge when 21 cars of the 80-car train derailed shortly before midnight. They were buried in coal and died.