The driver of the train that crashed in Spain, killing at least 80 people including one American, was detained and put under formal investigation Thursday after security video showed the train derailed after speeding around a tight curve.
Excessive speed has been identified as the likely main cause of the accident, official sources told Reuters as hospitals treated dozens of injured passengers, including at least five Americans.
President Barack Obama said in a statement that "the American people grieve with our Spanish friends."
Spain has declared three days of national mourning over the crash, Europe’s worst mainline rail accident for more than 25 years. The death toll has climbed to 80 and could rise further, Galicia regional officials told Reuters and AFP.
The horrific moment when the express train left the tracks on Wednesday night was captured on security video obtained by Spain’s El Pais newspaper.
Images from the scene showed bodies covered in blankets and towels lying next to toppled and crushed carriages as rescuers worked to pull survivors out of broken windows.
The driver - one of two in the cab at the time of the crash - has been put under formal investigation and was being held in custody in the hospital where he is being treated, the Supreme Court of the Galicia region said in a statement, according to Reuters.
"The judge has ordered the police to take a statement from the driver," the court said.
El Pais cited sources close to the investigation saying the driver stated immediately after the crash that he had been traveling at 118 mph on the curve, which had a speed limit of 49 mph.
Trapped in his wrecked cab, he reportedly told supervisors over the radio: "We're human! We're human," according to El Pais. "I hope there are no fatalities because they will fall on my conscience," he said, according to the newspaper’s source. NBC News was unable to immediately confirm the report.
A State Department official said it had been confirmed that a U.S. citizen was killed and five had been injured. "These numbers may change as we receive additional information."
Obama said he and his wife Michelle were “shocked and saddened” by the derailment.
“On behalf of the American people, we offer our deepest sympathies and condolences to the families and loved ones of the more than 80 people who lost their lives,” he said.
“We extend our wishes for a full recovery to those who were hurt. We also offer our heartfelt gratitude to the Government of Spain and to the rescue personnel who are working to locate the missing and treat the injured ... Today the American people grieve with our Spanish friends, who are in our thoughts and prayers.”
He added the United States was ready to provide “any assistance we can in the difficult days ahead.”
Pope Francis, who is currently visiting Brazil, sent his condolences and message of support to the bishop of Santiago de Compostela.
All the bodies had been removed from the wreckage by Thursday morning.
Mar Linares, 42, from the Galician city of La Coruna, told Reuters that her 15-year-old son Marco, who was travelling from Madrid on the train, was in intensive care.
"He was trapped by train wreckage but he managed to pull a hand free and that was how he was found. He says there was a lady on top of him who had been travelling with a little girl, and the lady was dead," she said at the hospital.
Tomas Lopez, whose wife and two children were travelling on the train, told Reuters that "the worst thing is the uncertainty, I feel desperate" as he searched for them at Santiago University Hospital.
"My daughter is OK but I don't know where my wife and son are. My wife brought them from Madrid to see museums and such... I have been looking for them all night from one place to another," he said, tears rolling down his face.
So many local residents lined up to donate blood that officials were forced to open additional donation points.
Spain's prime minister, Mariano Rajoy, declared three days of national mourning after visiting the site.
"The scene is shocking, it's Dante-esque," the head of Spain's Galicia region, Alberto Nunez Feijoo, said Wednesday night in a radio interview, according to Reuters.
Some 94 people were injured, of whom 35 were in a serious condition, including four children, the deputy head of the regional government said.
The crash, which happened at 8:41 p.m. local time (2:41 p.m. ET) Wednesday, was Europe’s deadliest mainline train accident in more than 25 years and Spain's worst in four decades.
It also cast a shadow of tragedy over the entire Galicia region, which had been due to celebrate a public holiday Thursday.
Santiago de Compostela had been preparing for the festival of St. James, when thousands of Christian pilgrims from around the world pack the streets. It is likely the train was packed full with people traveling for the holiday.
Officials said all of the celebrations, including a traditional High Mass at the city’s centuries-old cathedral, were canceled.
A group of volunteers waiting to give blood to help those injured in the train accident close to Santiago de Compostela, Galicia, Wednesday.
Spain’s rail network is one of the most modern and successful in Europe, following decades of public investment.
Transport expert and author Christian Wolmar said it was not clear if high-tech safety systems, which override inputs by the driver, would have been in use at the time.
“On high speed lines, the European Train Control System should automatically correct the speed of the train, but this accident may have happened on a stretch of line which is not designated as high-speed,” he said.
Rail workers’ union SEMAF expressed “support for the comrade who has been implicated in this accident” as well as “condolences” to the victims, according to RTVE.
The crash is Spain’s biggest disaster since the 2004 terror attack at Madrid’s Atocha station that left 191 dead, and its worst train crash since 1972 when a collision left 86 dead.
In November 2000, 155 people were killed when a fire in a tunnel engulfed a funicular train packed with skiers in Austria.
In Montenegro, up to 46 people were killed and nearly 200 injured in 2006 when a packed train derailed and plunged into a ravine outside the capital, Podgorica.
NBC News' Brinley Bruton and Jason Cumming and Reuters contributed to this report.