The accident happened when a Madrid-bound service run by private firm Iryo derailed and crossed onto the opposite track, smashing into an oncoming train operated by state company Renfe.
The stretch of track where a collision between two high-speed trains in Spain left 46 people dead in January broke the day before and went undetected, the Civil Guard said in a report on Wednesday.
The 18 January accident in the southern Spanish region of Andalusia was one of Europe's deadliest such catastrophes this century and prompted scrutiny of the safety of the country's railway network.
The accident happened when a Madrid-bound service operated by the private firm Iryo derailed and crossed onto the opposite track, smashing into an oncoming train operated by the state company Renfe.
A preliminary report published in January by Spain's railway accident investigatory committee CIAF suggested the track was cracked before the derailment.
The Spanish Civil Guard presented a new report to the court handling the investigation into the accident, in which the breakage is recorded as occurring at 9:46 pm on 17 January, Spanish media said, quoting the document.
Although "an electrical alteration consistent with a break" was registered, the signalling system "was not configured to automatically raise the alarm about it due to the lack of reliability," media quoted the report as saying.
The Civil Guard said the voltage variation from the evening of 17 January until the accident "is not usual," but that the technical possibility of receiving alerts about it "will have to be clarified."
The report also ruled out sabotage, terrorism and negligence by the train drivers.
The CIAF investigation expected to provide final conclusions is ongoing.
Meanwhile, the Andalusian regional government is studying the contents of the Civil Guard's report to assess possible legal action, while the Ministry of Transport has opened its own probe to determine why the break in the track did not trigger an alert.
The Spanish government said in January it would award €20 million in compensation to the families of the victims of the crash in Adamuz, which left 150 people injured.
For those injured in the crash, payments were expected to range from €2,400 to €84,000 depending on the severity of the injuries, Transport Minister Oscar Puente said.
