MADRID/UNITED NATIONS – Spanish Prime Minister Pedro Sanchez on Wednesday announced three days of national mourning following a collision between two high-speed trains in central Spain that killed at least 68 people, pledging a thorough investigation into the cause of the tragedy.

Speaking at an emergency command post near the accident site in Toledo province, Sanchez said the government would investigate the circumstances surrounding the collision and take firm action if negligence or failures in railway safety systems were found.

READ MORE: At least 50 killed, 179 injured in high-speed train collision in Spain

"We will get to the bottom of this," Sanchez said, stressing that any negligence would be uncovered and any failures in safety systems would be addressed decisively.

Sanchez said that "all of Spain is in mourning" following the tragedy, emphasizing that the government's immediate priority was to assist the victims and their families, rescue as many survivors as possible, and provide medical and psychological support to those affected.

Sanchez also expressed his condolences to the families of the victims, saying the entire country was in mourning.

The collision occurred on Wednesday morning in Toledo province, about 120 kilometers south of Madrid, when a Renfe high-speed train struck a stationary Iryo train from behind on the Madrid-Seville railway line.

According to Emiliano Garcia-Page, president of the autonomous community of Castilla-La Mancha, the death toll has risen to 68, while dozens of people have been injured. The exact cause of the accident remains under investigation. 

Transport minister cites 'inexplicable' decision as likely cause

Also on Wednesday, Spanish Transport Minister Oscar Puente pointed to an "inexplicable" decision by a railway control center to authorize the Renfe train to resume its journey despite a red signal as a possible cause of the deadly high-speed train collision.

Speaking at a press conference in Madrid, Puente said that a computer communication failure had disabled the LZB railway safety control system, automatically bringing trains along a 63-km section of the line to a halt.

However, the control center subsequently authorized the Renfe train to resume its journey despite a red signal prohibiting it from proceeding.

Puente described the decision to authorize the train's movement while another train remained stationary on the track as "inexplicable."

UN chief saddened by train crash

At the United Nations, UN Secretary-General Antonio Guterres was deeply saddened and shocked at learning of the train accident in Spain, his spokesman said Wednesday.

Guterres extended his deepest condolences to the families of the victims, as well as to the people and government of Spain, said Stephane Dujarric, the spokesman.