1.   Russian officials are investigating whether it was involved in a collision with an unidentified vessel, or perhaps struck an old mine or was scuttled by some internal defect.

2.   Investigators were considering the possibility of whether an internal defect, called transverse fissure, may have occurred when the steel in the rail was forged.

3.   Investigators were considering the possibility of whether an internal defect, called transverse fissure, may have occurred when the steel in rail was forged.

4.   That stretch of track is visually inspected three times a week, and is examined monthly with an ultrasound device that can discover internal defects called transverse fissures.

5.   The straightaway stretch of track is visually inspected three times a week and inspected monthly with an ultrasound device designed to uncover internal defects called transverse fissures.