At least 40 people were killed and dozens injured after two high-speed trains collided in southern Spain, local authorities said.

The accident, near the city of Cordoba, has been described by local authorities as Spain’s worst rail disaster in more than a decade.

Prime Minister Pedro Sánchez visited the scene on Monday and announced a three-day mourning period.

Here’s what we know so far about the incident.

The accident occurred at around 19:45 local time (18:45 GMT) on Sunday, about an hour after one of the trains left Malaga for Madrid.

Driver Adif said the train derailed and crossed onto the tracks on the opposite side.

Spain’s Transport Minister Óscar Puente announced on Sunday that the train collided with an oncoming train traveling from south of Madrid to Huelva, sending the train into an embankment along the railway tracks.

Most of the casualties were in the first car of the Huelva-bound train, he added.

The cause of the train derailment remains unclear.

Officials said an investigation has been launched but it is expected to take at least a month to determine what happened.

Mr Puente described the accident as “very strange” and said all railway experts consulted by the government were “very perplexed by this accident”.

Renfe, the president of Spain’s state-run railway operator, said he had “discarded” the possibility that the accident was caused by excessive speed or human error.

Álvaro Fernández Heredia told Spanish state radio RNE that even if there was a mistake, the train’s systems would correct it.

He added that both trains were traveling below their maximum speed on the section of track where the accident occurred.

Fernández Heredia suggested that mechanical failure or infrastructure issues were likely the cause.

Meanwhile, at a press conference during his visit to Adams, Prime Minister Sánchez vowed to find out the cause of the crash and thanked emergency workers who helped “at such a moment of pain and tragedy.”

According to a Renfe statement, the two trains operated by Iryo and Renfe had about 400 passengers and crew members on board.

It is unclear whether anyone is still trapped inside the carriage, but rescue teams have arrived at the scene.

“The problem is that the carriage is twisted and the metal is twisted along with the people inside,” Francisco Carmona, head of Cordoba’s fire brigade, told Spanish public broadcaster RTVE.

“We have had to move dead people in order to contact the living. It is a difficult and delicate task,” he added.

Andalusian regional government president Juanma Moreno told local media outlet Canal Sur that he was waiting for “heavy machinery” to “virtually lift” parts of the second train, which “represented the worst part of this accident.”

He added that emergency services would not be able to begin “searching and identifying” the remaining victims “until heavy equipment has done its job and the wagons have been freed from the tracks.”

The 39 victims of the crash have not yet been identified, and Puente said an investigation into the crash had begun, but the death toll was “not yet final.”

Local emergency services said 122 people were receiving medical assistance as of Monday afternoon, of whom 48 remained in hospital.

Of the 48 victims still in hospital, five are under the age of 18.

Moreno said teams are working to identify the deceased.

Passengers on board the Madrid-bound train described the moment of impact as feeling like an “earthquake”, saying the train’s windows were broken, luggage was shifted and people were thrown to the floor.

“I was in the lead car and there was a seismic moment and the train actually derailed,” journalist Salvador Jiménez told Canal de Sur.

“People were screaming and calling for doctors,” he added.

Another passenger, Lucas Meliaco, told Spanish broadcaster La Sexta Noticias that he was in the fifth car of the same train when he started “feeling a crashing sound” that got louder and louder.

“When another train passed us, everything started vibrating. There was a jolt behind us and it felt like the whole train was going to fall apart,” he explained.