Most of those killed and injured were in the front carriages of the second train, which was travelling south from Madrid to Huelva
Rail company Iryo says the "relatively new" Madrid train, built in 2022, was inspected four days ago .Meanwhile, Transport Minister Oscar Puente says the crash happened on a straight part of the track, calling it "extremely strange"
The president of state-funded firm Renfe, which operated one of the trains involved in the collision, says he has "discarded" the possibility that the incident happened due to excessive speed or human error.
Speaking to RNE, Spain's national radio, Álvaro Fernández Heredia says human error is "practically discarded" as even if a mistake had been made, a system within the train would have fixed it.
He adds that both trains were under the 250km/h (155mph) maximum speed on the stretch of track where the crash happened, and that it took place on a straight section of the track rather than a curve.
Fernández Heredia points to either a mechanical fault or infrastructure issue as a more probable cause, but emphasizes that it's "very complicated" to arrive at a conclusion so soon after the incident.
Lucas Meriako, a passenger on the high-speed Madrid-bound train that derailed and collided with another oncoming train, says the experience was "horrifying".
"We were in carriage five and we started to feel some banging on the track, nothing unusual," he tells Spanish news broadcaster, La Sexta Noticias.
Then, the banging became louder, he says.
"Another train passed us and everything started vibrating. There was a jolt behind us and the feeling that the whole train was going to fall apart," he describes.
Meriako says that the impact of the collision shattered the train's windows, displaced luggage and threw people on to the floor. People then started breaking windows to get out.
Spain's Transport Minister Óscar Puente tells reporters the crash took place on a straight section of track that had been completely renovated last May after major investment.
The first train to derail was also "practically new", he says.
This made the accident "extremely strange" and "very difficult to explain". Rail experts are "very surprised by this accident", he adds.
He says it will take at least a month for the results of the investigation to be released.
The company that operated the train that first derailed while travelling between Málaga and Madrid on Sunday night says the locomotive was inspected just four days ago.
In a statement quoted by AFP news agency, Iryo says the train was built in 2022 and so was relatively new.
It "veered onto the adjacent track for still unknown reasons", the company says.
Carriages on the Iryo-oeprated train derailed and crossed over to the opposite tracks in Adamuz on Sunday night, colliding with an oncoming train travelling south between Madrid and Huelva.
Around 100 people were believed to have been onboard this train, operated by state-funded company Renfe
Details are still emerging from last night's train crash, with emergency services still working to establish the facts. Here's what we know so far:
At least 39 people have been killed - although Spain's transport minister says this number "is not yet final"
Emergency services have treated 112 people, 48 of which remain in hospital, including five children. Eleven of those still in hospital are in intensive care
The crash happened at 19:45 local time (18:45 GMT) as carriages on a Madrid-bound train derailed and crossed over to the opposite tracks, colliding with an oncoming train near Córdoba, in southern Spain
Around 300 people - passengers and staff - were on board the high-speed train operated by Iryo, which derailed. One hundred others were on the other train, operated by state-funded firm Renf
What caused the train to derail remains unclear. Officials say an investigation has been launched but is not expected to determine what happened for at least a month
Spanish Transport Minister Óscar Puente has described the incident as "extremely strange", adding that all the railway experts consulted by the government "are extremely baffled by the accident"