In an interview with The Associated Press, an injured American passenger said he saw on a TV monitor screen inside his car that the train was traveling 194 kph (121 mph) seconds before the crash - far above the 80 kph (50 mph) speed limit on the curve where it derailed.
Police lowered the death toll from 80 to 78 as forensic scientists matched body parts at a makeshift morgue set up in a sport arena in Santiage de Compostela, the train's destination and a site of Catholic pilgrimage preparing to celebrate its most revered saint.
Investigators have opened a probe into possible failings by the 52-year-old driver and the train's internal speed-regulation systems.
The driver, Francisco Jose Garzon Amo, was officially detained in the hospital where he was recovering, said Jaime Iglesias, the National Police chief of the Galicia region. He is being question "as a suspect for a crime linked to the cause of the accident," Iglesias said.
The driver is under guard by police and cannot yet testify because of his condition, Iglesias said, adding that he did not have details on his status but that it could delay efforts by police to question him.
Police are still working to identify what they believe are the remains of six people, and that the death toll count could change as they continue their work matching body parts, said Antonio De Amo, the police chief in charge of the scientific service for Spain's National Police.
An American victim was identified by the Diocese of Arlington in northern Virginia as Ana Maria Cordoba. Also among the dead were an Algerian and a Mexican, Spanish police said Friday.
Investigators, meanwhile, have taken possession of the train's "black box" and will hand it over to the investigating judge, Iglesias said. The box has not been opened yet, he said.
The box records the train's trip data, including speed, distances and braking, and is similar to a flight recorder for an airplane. A court spokeswoman Maria Pardo Rios declined comment on how long analysis of the box's contents would take.
Meanwhile, Stephen Ward, 18-year-old Mormon missionary from Utah, who was on the train said he was writing in his journal when he looked up at the monitor and saw the train's speed. Then, he said, "the train lifted up off the track. It was like a roller coaster."
Seconds later, Ward remembered, a backpack fell from the rack above him and he felt the train fly off the track. That was his last memory before he blacked out.
When Ward woke up, someone was helping him walk out of his train car and to crawl out of a ditch where the car had toppled over. He thought he was dreaming for 30 seconds until he felt his blood-drenched face and noticed the scene around him.
"Everyone was covered in blood. There was smoke coming up off the train," he said. "There was a lot of crying, a lot of screaming. There were plenty of dead bodies. It was quite gruesome."