At least 41 people are now known to have died in Sunday night’s collision between two Spanish high speed trains, and investigators are focusing on the possibility that a fault in a rail joint could have been to blame. Rescuers have been continuing to search the wreckage for more casualties.

The front cars of a high speed train bound for Madrid became derailed in Córdoba province and collided with a train travelling in the opposite direction, pushing part of it into an embankment. The train to Madrid was being run by open access operator Iryo, while the other was a Renfe service.

Reports are speaking of a ‘gap’ in a rail, perhaps caused by a faulty weld, although it has also been suggested that such a gap could be damage resulting from the derailment itself.

It has also been reported that the drivers’ union SEMAF expressed concern about the state of the track as long ago as last August.

News agency Reuters said it has seen a letter from the union to the infrastructure operator Adif, which passed on complaints from drivers about ‘potholes’ in the track and overhead line problems, without result. Adif, which is the equivalent of Network Rail in Britain, told the newspaper El Mundo that it had recorded almost a dozen incidents on this section, and one report spoke of a member of train crew being injured by a piece of equipment which had been dislodged by ‘jolting’.

Meanwhile, inspectors from the Spanish Commission of Investigation of Rail Accidents have been gathering evidence. The high speed line from Madrid to Andalucía is expected to remain closed until the end of the week at least.

Prime minister Pedro Sánchez has declared three days of national mourning and promised a full investigation to reporters in Adamuz, a small town near the accident site. He continued: ‘We will uncover the answer, and once the cause of this tragedy is determined, we will present it with absolute transparency.’

The accident is the worst on Spanish railways since 2013, when a high speed train became derailed near Santiago and collided with a bridge. The cause was confirmed to be overspeeding.

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