Investigators said they would seek evidence of failings by Francisco Jose Garzon Amo, the 52-year-old driver, as well as the train's internal speed-regulation systems in Wednesday's derailment.
The train company, Renfe, defended the driver Friday, lauding what it called his exhaustive experience. But the country's railway agency, Adif, noted that the driver should have started slowing the train long before reaching the disastrous turn.
In an interview with The Associated Press, an American passenger injured on the train said he saw on a TV monitor screen inside his car that the train was traveling 121 mph seconds before the crash — far above the 50 mph speed limit on the curve where it derailed. The passenger, 18-year-old Stephen Ward, said the train appeared to have accelerated, not decelerated.
And Gonzalo Ferre, president of the rail infrastructure company Adif, said the driver should have started slowing the train 2.5 miles before reaching a dangerous bend that train drivers had been told to respect.
"Four kilometers before the accident happened he already had warnings that he had to begin slowing his speed, because as soon as he exits the tunnel he needs to be traveling at 80 kilometers per hour," Ferre said.
At the scene, hundreds of onlookers watched as crews used a crane Friday to hoist smashed and burned-up cars onto flat-bed trucks to cart them away. The shattered front engine had been tipped back upright but remained resting beside the tracks, just yards from the passage of resumed train traffic.
Grieving families gathered for funerals near the site of the crash in Santiago de Compostela, a site of Catholic pilgrimage that had been preparing to celebrate its most revered saint, James. Those annual festivities planned for Thursday were canceled.
Police lowered the death toll Friday to 78 as forensic scientists matched body parts. They previously had identified 80 dead.
Amo was arrested Thursday night in the hospital. Photographs indicated he had suffered a head wound in the crash.
Jaime Iglesias, police chief of Spain's northwest Galicia region, said Amo would be questioned "as a suspect for a crime linked to the cause of the accident." When asked, Iglesias described Amo's alleged offense as "recklessness." He declined to elaborate.
The driver is under police guard but has yet to be interviewed. That might be delayed because of his medical treatment, Iglesias said.
Renfe said Amo is a 30-year employee of the state train company, who became an assistant driver in 2000 and a fully qualified driver in 2003.
Amo had driven trains past the spot of the accident around 60 times and "the knowledge of this line that he had to have is exhaustive," Renfe's president, Julio Gomez-Pomar, said in a TV interview.
Iglesias said police took possession of the train's "black box," which is expected to shed light on why it was going faster than the speed limit. The box will be handed over to the investigating judge, Iglesias said, adding that the box had not been opened yet.
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 declined to comment on how long analysis of the box's contents would take.
Police said they had positively identified 75 bodies but still required help from relatives and DNA testing to identify what they believe are the remains of three people.
Spanish authorities said the dead included at least five foreign visitors: one person each from Algeria, the Dominican Republic, Italy, Mexico and the United States.
Catholic Church authorities in the U.S. state of Virginia identified the dead American as Ana Maria Cordoba, 47. She had been traveling to Santiago de Compostela to meet up with her youngest son, also named Santiago, who had just completed the area's celebrated religious trek through the mountains of northern Spain.
The New York Daily News reported that her husband and daughter also were on the train but survived, with the husband sustaining skull injuries and the daughter a broken leg.
The Dominican government identified its victim as Ynoa Rosalina Gonzalez, 42, a senior official in the country's Economy Ministry. It said Gonzalez' two sisters provided DNA samples to confirm the identity of the body.
Eyewitness accounts backed by security-camera footage of the moment of disaster showed that the eight-carriage train was going too fast as it tried to turn left underneath a road bridge. After impact, witnesses said a fire engulfed passengers trapped in at least one carriage, most likely driven by ruptured tanks of diesel fuel carried in the forward engines.
Ward, a 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 registering as 194 kph. Then, he said, "the train lifted up off the track. It was like a roller coaster."
Ward said he remembered a backpack falling from the rack above him as his last memory before he blacked out.
When Ward woke up, someone was helping him walk out of his train car and crawl out of a ditch where the carriage 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."
It was Spain's deadliest train accident since 1972, when a train collided with a bus in southwest Spain, killing 86 people and injuring 112.