MADRID — Spain began three days of mourning on Tuesday as rescuers continue to comb through the wreckage of twisted train cars and scattered debris to locate victims after a train collision that killed at least 40 people and injured dozens.

A high-speed train carrying about 300 Madrid-bound passengers derailed and collided with an oncoming train Sunday evening in Spain’s worst rail disaster in more than a decade.

Spanish Prime Minister Pedro Sanchez, who announced three days of national mourning, promised to get to the bottom of why the two trains collided.

Rail network operator Adif said the collision happened at 19:45 local time (18:45 GMT) on Sunday, about an hour after one of the trains left Málaga heading north to Madrid, when it derailed on a straight stretch of track near the city of Córdoba.

The force of the crash pushed the carriages of the second train into an embankment, according to Transport Minister Óscar Puente. He added that most of those killed and injured were in the front carriages of the second train, which was travelling south from Madrid to Huelva.

Rescue teams said the twisted wreckage of the trains made it difficult to recover people trapped inside the carriages.

Sanchez visited the site of the crash with senior officials on Monday afternoon.

"This is a day of sorrow for all of Spain, for our entire country," he told reporters.

"We are going to get to the truth, we are going to find the answer, and when that answer about the origin and cause of this tragedy is known, as it could not be otherwise, with absolute transparency and absolute clarity, we will make it public."

Puente said an investigation could take at least a month, describing the incident as "extremely strange".

But Reuters news agency quoted an unnamed source briefed on initial investigations as saying experts had found a faulty joint on the rails, which was causing a gap between rail sections to widen as trains travelled over it. They added that the joint was key to identifying the cause of the accident.

Spain's El País newspaper said it was not clear whether the fault was a cause or a result of the crash.

Four hundred passengers and staff were on board the two trains, the rail authorities said. Emergency services treated 122 people, with 41, including children, still in hospital. Of those, 12 are in intensive care.

Puente said the death toll "is not yet final". Officials are working to identify the dead.

The type of train involved in the crash was a Freccia 1000, which can reach top speeds of 400 km/h (250 mph), a spokesperson for the Italian rail company Ferrovie dello Stato told Reuters.

All high-speed services between Madrid and the southern cities of Malaga, Cordoba, Sevilla and Huelva have been suspended until Friday.

King Felipe VI and Queen Letizia said they were following news of the disaster "with great concern" and offered their "most heartfelt condolences".

The emergency agency in the region of Andalusia urged any crash survivors to contact their families or post on social media that they are alive.

The Spanish Red Cross has deployed emergency support services to the scene, while also offering counselling to families nearby.

Spain's high-speed rail network is the second largest in the world, behind China, connecting more than 50 cities across the country. — Agencies