Spain began three days of mourning Thursday after a high-speed train crash near Toledo killed at least 68 people, BBC and ABC Australia report. Questions are growing about how two trains ended up on the same stretch of track.

A train run by state operator Renfe, carrying 397 passengers, struck the back of a stopped train run by private firm Iryo on Wednesday morning, the BBC reports. The Iryo train had 147 people on board. Several carriages fell down an embankment.

Transport Minister Óscar Puente said a computer failure knocked out signals along about 39 miles of track at about 8:30 am. A control center in Madrid then took over. Puente said it made an "inexplicable" decision to let trains move again even though a stop signal was active.

ABC Australia reports, citing the Spanish paper El Mundo, that Iryo says it never got an order to move and kept its train stopped. The Renfe train was ordered to resume at normal speed, ABC Australia reports. The BBC says recordings of talks between controllers and drivers appear to show one train was cleared to move while another ahead had stalled.

The CSIF union called for Puente to resign and said a worker was being made a scapegoat. Rail drivers planned a strike Thursday "in defence of railway safety," the BBC reports. Police used tear gas on protesters outside the Transport Ministry overnight.

It is Spain's second deadly high-speed crash this year. In January, 46 people died near Córdoba on the same Madrid to Seville line. Prime Minister Pedro Sánchez called the two crashes "unacceptable."

ABC Australia reports the stretch was being moved to a newer European signaling system.

What we know: At least 68 died when a Renfe train hit a stopped Iryo train after a signal failure.
What we don't: Who cleared the Renfe train to move, and how fast it was going.