At least 68 people have been confirmed dead in the collision, but the death toll is likely to rise. Relatives are now desperately seeking information about their loved ones. Rescue services also searched the train wreckage for more victims overnight and on Thursday morning.
"Rescue workers and the military task force are still working with great caution, as we cannot yet be sure that there are no more victims," regional spokeswoman Esther Padilla told national broadcaster TVE.
The accident happened when a high-speed train travelling from Madrid to Seville collided with another high-speed train that was stationary on the tracks. It is the second fatal high-speed train accident in the country this year.
On Thursday, Spain began a three-day national mourning period declared by the government.
Clashes broke out overnight outside the transport ministry in the Spanish capital between far-right groups demanding the government's resignation after the accident and police, who used tear gas against the protesters.
Prime Minister Pedro Sánchez has promised a thorough investigation into Wednesday's accident. Transport Minister Óscar Puente said it was caused by human error: an "inexplicable" order was given from a central control centre for Spain's trains after a safety system malfunctioned.
