Spain began three days of national mourning on Tuesday for 41 people killed after two high-speed trains collided in the country's deadliest rail disaster in over a decade. 

The collision took place late Sunday when a high-speed train operated by rail company Iryo, travelling from Malaga to Madrid, derailed near Adamuz in the southern Andalusia region.

It crossed onto the other track, where it crashed into an oncoming train, which also derailed.

The cause of the crash remains a mystery, though investigators say they will focus on coach six of the Iryo train, which was the first to derail. The Guardia Civil has asked for the carriage to remain on the tracks so it can be examined. 

Rail experts "extremely surprised" by crash on straight track

"There are many pieces of the puzzle we must fit together," Spain's Transport Minister Oscar Puente told the country's Onda Cero radio network.

Puente called the crash "extremely strange" on Monday because it happened on a flat stretch of track that had been renovated in May, adding that "all the railway experts are extremely surprised."

"Nothing seems to suggest anything was wrong with it," he said Tuesday, adding that freshly laid track can sometimes exhibit "youthful sins."

Spain's chief high-speed train engineer, Álvaro Aznar, told CBS News on Monday that the "earliest signs point to mechanical failure" as the cause of the crash.

Death toll could still climb as recovery work continues

The death toll rose to 41 after the body of another passenger was recovered Monday evening from one of the Iryo train carriages, the regional government said. More than 120 people were injured, and 39 were still hospitalized on Tuesday, including four children, it added.

Flags flew at half-mast on public buildings, television anchors wore black, and cabinet ministers curtailed public appearances as Spain observed the first of three days of national mourning. Members of the Assembly of Extremadura, the neighboring region, held a minute's silence for the victims of the accident Tuesday morning.

Heavy machinery was deployed Monday to lift the most severely damaged train carriages and give rescuers better access.

The head of Andalusia's regional government, Juan Manuel Moreno, warned Monday that it would take another 24-48 hours "to know with certainty how many deaths have resulted from this terrible accident."

King Felipe VI and Queen Letizia were scheduled to meet with rescuers and officials in Adamuz later on Tuesday.