MADRID, — Investigators are working to determine how a high-speed passenger train came to slam into the back of another train stopped on the tracks south of Madrid on Wednesday, killing at least 68 people in one of Spain's deadliest rail disasters.

The cause has not been officially established. A court in Orgaz, in Toledo province, and Spain's Railway Accident Investigation Commission, known as CIAF, have opened investigations, and neither has reached conclusions. But statements from officials and reports in the Spanish media point to a chain of failures involving signaling equipment, communications and decisions at a rail control center.

The collision happened at about 8:54 a.m. near Urda, on the high-speed line between Madrid and Seville. A Renfe AVE train struck a stationary train operated by private company Iryo. According to an updated count from Spain's 112 emergency service, at least 68 people were killed and 77 injured. Castilla-La Mancha regional President Emiliano García-Page said four bodies remained trapped in the wreckage and the toll could rise.

These are the key factors under scrutiny.

1. A failure in the main signaling system

The trouble began with a fault in LZB, the automatic train protection system that controls speed and signaling on the Madrid-Seville line. According to transcripts of radio exchanges published by broadcaster Cadena SER and news site elDiario.es, the Renfe driver reported an LZB failure at about 8:31 a.m., about 23 minutes before the crash. The transcripts have not been independently verified.

Several trains were stopped during the malfunction,........

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