An Associated Press analysis of video images suggests the train may have been traveling 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 Wednesday night on its approach to this Christian festival city in northwest Spain, where planned celebrations in honor 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 hospitalized in six regional hospitals, 31 of them - including four children - in critical condition.
The U.S. 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.
In the morning, 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 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 80 kilometers (50 miles) per hour.
An Associated Press estimate of the train's speed at the moment of impact using the time stamp of the video and the estimated distance between two pylons gives a range of 144-192 kph (89-119 mph). Another estimate calculated on the basis of the typical distance between railroad ties gives a range of 156-182 kph (96-112 mph).
The video footage, which the Spanish railway authority Adif said probably came from one of its cameras, shows the train carriages starting to buckle soon into the turn.
Murray Hughes, consultant editor of Railway Gazette International, said it appeared that a diesel-powered unit behind the lead locomotive was the first to derail. The front engine itself quickly followed, violently tipping on to its right side as it crashed into a concrete security wall and bulldozed along the ground.
In the background, all the rear carriages could be seen starting to decouple and come off the tracks. The picture went blank as the engine appeared to crash directly into the camera.
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.
"I saw the train coming out of the bend at great speed and then there was a big noise," one eyewitness who lives beside the train line, Consuelo Domingues, told The Associated Press. "... Then everybody tried to get out of the train."
Santiago officials had been preparing for the city's internationally celebrated Catholic festival Thursday but canceled it and took control of the city's main indoor sports arena to use as a makeshift morgue. There, relatives of the dead could be seen sobbing and embracing each other.
The Interior Ministry, responsible for law and order, ruled out terrorism as a cause.
While sections of the Spanish press pointed an accusatory finger at the train driver, Spanish authorities and railway safety experts cautioned that a fault in systems designed to keep trains traveling at safe speeds could be to blame.
Spain's lead investigator in the crash, Judge Vazquez Taín, ordered detectives to question the train driver.
Renfe identified the driver, Francisco Jose Garzon Amo, as a 30-year employee of the state rail company who became an assistant driver in 2000 and a fully qualified driver in 2003. The company said Amo took control of the train from a second driver about 100 kilometers (65 miles) south of Santiago de Compostela.
Renfe's president, Julio Gomez-Pomar Rodriguez, president of Renfe, told Spain's Cadena Cope radio network that the driver had worked on that route for more than one year.
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.
"July 24 will no longer be the eve of a day of celebration but rather one commemorating one of the saddest days in the history of Galicia," said Alberto Nunez Feijoo, regional president of Galicia. Santiago de Compostela is its capital.
The accident created a scene that was "Dante-esque," Feijoo said. He said Galicia would observe seven days of mourning.
Rescue workers spent the night searching through smashed carriages alongside the tracks.
As dawn broke, cranes brought to the scene were used to lift the carriages away from the tracks. Rescue workers collected passengers' scattered luggage and loaded it into a truck next to the tracks.
Rescuers described a scene of horror immediately after the crash. Smoke billowed from at least one carriage that had caught fire, while another had been torn into two parts.
Residents of the residential neighborhood closest to the rail line struggled to help victims out of the toppled cars. Some passengers were pulled out of broken windows. Television images showed one man atop a carriage lying on its side, using a pickaxe to try to smash through a window. Other rescuers used rocks to try to free survivors from the fiery wreckage.
Nearby, rescue workers lined up bodies covered in blankets alongside the tracks.
State-owned train operator Renfe said the crash happened at 8:41 p.m. (1841 GMT) about 2.5 miles (4 kilometers) south of Santiago de Compostela.
Spanish media said the train had two drivers aboard and both survived.
However, Galician court officials said the train had only one driver. Court spokeswoman Maria Pardo Rios said the driver survived and was expected to give a statement to police later Thursday. She declined to name the driver.
Renfe said it and Adif, another state-owned company that manages tracks, signals and other railway infrastructure, were cooperating with a judge appointed to investigate the accident.
It was the world's third major rail accident this month.
On July 12, six people were killed and nearly 200 were injured when four cars of a passenger train derailed south of Paris.
On July 6, 72 cars carrying crude oil derailed in Lac-Megantic, Quebec, setting off explosions and fires that killed 47 people.
Catholic pilgrims converge on Santiago de Compostela annually to celebrate a festival honoring St. James, a disciple of Jesus whose remains are said to rest in a shrine. The city is the main gathering point for those who reach the end of the El Camino de Santiago pilgrimage route that has drawn Christians since the Middle Ages. Several injured passengers said they felt a strong vibration just before the cars jumped the tracks, according to Xabier Martinez, a photographer who talked with them after arriving at the scene as rescue workers were still removing bodies.
One passenger, Ricardo Montero, told the Cadena Ser radio station that "when the train reached that bend it began to flip over, many times, with some carriages ending up on top of others, leaving many people trapped below. We had to get under the carriages to get out."
Another passenger, Sergio Prego, told Cadena Ser the train "traveled very fast" just before it derailed and the cars flipped upside down, on their sides and into the air.
"I've been very lucky because I'm one of the few able to walk out," Prego said.
The Alvia 730 series train started from Madrid and was scheduled to end its journey at El Ferrol, about 95 kilometers (60 miles) north of Santiago de Compostela. Alvias operate high-speed services but do not go as fast as Spain's fastest bullet trains, called AVEs. The maximum Alvia speed is 250 kph (155 mph) on tracks made especially for the AVEs, and they travel at a maximum speed of 220 kph (137 mph) on normal gauge rails.
Other Spanish train calamities include a 1944 accident involving three trains that crashed in a tunnel. That disaster produced wildly disputed death tolls ranging from the government's official count of 78 to more than 500, according to later research.
In 2006, 43 people died when a subway train crashed because of excessive speed in the southern city of Valencia. In 2004, 191 died when al-Qaida-inspired terrorists detonated 10 bombs on four Madrid commuter trains.
Associated Press writers Alan Clendenning, Ciaran Giles and Harold Heckle in Madrid, Panagiotis Mouzakis, Fisnik Abrashi and Robert Barr in London, Deb Riechmann in Washington, and Shawn Pogatchnik in Dublin contributed to this report.