A Spanish train that hurtled off the rails and smashed into a security wall as it rounded a bend was going so fast that carriages tumbled off the tracks like dominos, killing 80 people and maiming dozens more, according to eyewitness accounts and video footage.
An analysis of video images suggests the train may have been travelling at twice the speed limit, or more, along that curved stretch of track. The unanswered question is: Why?
Spain's government said two probes have been launched into the train's derailment on its approach to a Christian festival city in northwest Spain, where planned celebrations in honour of one of Jesus' disciples gave way to a living nightmare.
The regional government in Galicia confirmed that police planned to question the 52-year-old train driver, in Santiago de Compostela's main hospital with unspecified injuries, as both a witness and as a possible suspect, but cautioned that possible faults in safety equipment were also being investigated.
The Interior Ministry raised the death toll to 80 in what was Spain's deadliest train wreck in four decades. The Galician government said 94 others remained in six regional hospitals, 31 of them - including four children - in critical condition.
The US State Department said one American was killed in the crash and five others were injured. State Department spokeswoman Marie Harf said those numbers were "likely to change" and declined to elaborate.
"Today the American people grieve with our Spanish friends, who are in our thoughts and prayers," President Barack Obama said in a statement.
On Thursday morning, Spain's prime minister Mariano Rajoy, a native of Santiago de Compostela, toured the crash scene alongside rescue workers and went to a nearby hospital to visit those wounded and their families. In the evening Spain's head of state, King Carlos, and Queen Sofia went to the same hospital, dressed in funereal black.
"For a native of Santiago, like me, this is the saddest day," said Mr Rajoy, who declared Spain would observe a three-day period of mourning. He said judicial authorities and the Public Works Ministry had launched parallel investigations into what caused the crash.
Eyewitness accounts backed by security-camera footage of the moment of disaster suggested that the eight-carriage train was going too fast as it tried to turn left underneath a road bridge. The train company Renfe said 218 passengers and five crew members were on board. Spanish officials said the speed limit on that section of track is 50 miles per hour.