Police have formally detained the driver of a train that derailed in northwestern Spain, killing at least 78 passengers and injuring another 130.
The National Police Chief for the Galicia region, Jaime Iglesias, said driver Francisco Jose Garzon Amo was arrested in the hospital where he is recovering.
Mr Iglesias said Garzon would be questioned "as a suspect for a crime linked to the cause of the accident".
Asked at a news conference in Santiago de Compostela why the driver was being detained, Mr Iglesias added: "For recklessness."
Mr Garzon boasted about speeding on his Facebook page
The 52-year-old driver is being guarded by police and cannot yet testify because of his medical condition, the police chief said.
He did not have any further details of his state of health but said it could delay his statement.
Sky News understands Garzon's condition is not serious. Pictures and video footage have emerged of him being led away from the crash scene with his head covered in blood while other images show the engine cabin was largely intact compared to the rest of the train.
Police also said the number of dead, yesterday thought to be 80, has been reduced to 78 as forensic science units continue to identify remains.
The train's black box recorder has been retrieved from the wreckage. Early indications suggested the train may have been travelling at more than twice the speed limit at the time of the crash on Wednesday night.
An official inspects the train's engine
The eight-carriage train came off the tracks on a bend, hit a wall and caught fire just outside the pilgrimage destination Santiago de Compostela in northwestern Spain.
The train entered the bend at 190km per hour (120mph), according to local media reports. The speed limit on the curve was 80km per hour (50mph).
As the investigation turned its focus more and more towards human error, the Spanish press revealed that immediately after the derailment Garzon allegedly said to officials at the railway station 3km from the crash: "I ****** up, I want to die. So many people dead, so many people dead."
Witnesses are also reported to have heard the driver shout into a phone: "I've derailed! What do I do?"
And as his country mourned, it emerged that Garzon is thought to have boasted on his Facebook page about how fast he was driving a train in March last year.
Staff from a hospital treating the injured hold a minute's silence
The driver posted a picture of a train speedometer at 200km per hour (124mph) on the social networking site. His Facebook page has since been blocked.
"What a blast it would be to go parallel with the Guardia Civil (Spanish police) and go past them triggering the radar. Haha what a fine for Renfe (Spanish rail operator) haha," he wrote on what is believed to be his Facebook page.
There was a second driver on the train, but it is believed Garzon was the only driver at the time.
He is understood to have taken control of the train from a second driver about 65 miles (104km) south of Santiago de Compostela.
According to reports, one of the drivers realised what was about to happen before the crash and made a desperate call to Renfe ahead of the bend, saying: "I'm going at 190km per hour, I'm going to derail."
A truck transports a carriage away from the scene of the crash
In a second call to Renfe after the accident, the driver explained that he was trapped in the train.
"We are human, we are human," he is reported to have said. "I hope there are no dead because they would fall on my conscience."
Two investigations are being carried out into the catastrophe - one to look into possible failings by the driver and the other to examine the train's in-built speed regulation systems and see if it was a technical malfunction that meant the driver was not warned of the reduced speed limit around the bend.
Many questions remain unanswered about what went wrong, with some experts claiming that high speed alone would not explain the crash and speculation that the train's braking systems might have failed.
Reporting from the scene, Sky News Europe Correspondent Robert Nisbet said investigators would be focusing on the statements made by the driver immediately after the derailment.
An aerial view of the carnage caused by the derailment
Nisbet pointed to further reports from Spanish media suggesting the driver had told officials at Santiago de Compostela train station that an electronic warning signal was flashing inside the driver's cabin to indicate he was going too fast.
"He is said to have pressed a button to acknowledge the warning but still apparently did not slow down," Nisbet said.
State train company Renfe said Garzon had been at the firm for 30 years and he had been driving trains for more than a decade.
He became an assistant driver in 2000 and a fully qualified driver in 2003.
Garzon is understood to have been on the Madrid to Ferrol service, on which the crash happened, for a year, after undergoing training specific to that line.
The impact was so huge one carriage flew several metres into the air
Meanwhile, with 72 of the 78 dead so far identified, medical experts are continuing to try to identify the six remaining victims as distraught families continue to wait for news about their loved ones.
DNA tests are expected to be carried out on those with catastrophic injuries, with results available in the coming days.
The victims of the crash included a US citizen and a Mexican. At least one British citizen and four children were among the 130 people injured. Just over 30 of those in hospital are still critically ill.
Spanish newspaper El Pais has revealed details of a dramatic WhatsApp conversation between one of the survivors trapped in the train wreck and her husband.
At 8.45pm local time, the woman sent messages saying she had been in an accident and was "crushed".
Relatives of passengers on the train wait for news of their loved ones
After what her husband described as "the longest five minutes of my life", she sent another message saying "I'm safe".
The 46-year-old woman escaped with minor injuries to her legs and has already been discharged from hospital.
Video footage from a security camera showed the train, which had 247 people on board, hurtling into a concrete wall at the side of the track.
The impact was so huge one carriage flew several metres into the air and landed on the other side of a concrete barrier.
The Alvia 730 series train was travelling from Madrid to the port city of Ferrol when it crashed about 8.40pm local time - 7.40pm UK time - on Wednesday.
Spanish Prime Minister Mariano Rajoy visits the scene of the crash
The crash occurred on the eve of a major Christian religious festival honouring St James, the disciple of Jesus whose remains are said to rest in a shrine.
Many of the dead or injured were believed to be Catholic pilgrims converging on the city.
Spain's Prime Minister Mariano Rajoy, who was born in Santiago de Compostela, visited the scene of the crash on Thursday and declared three days of official mourning across the country.
King Juan Carlos also visited one of the hospitals where many passengers are being treated.
Local reaction to the crash has been such that an appeal for blood donations resulted in hospitals having to turn people away because they could not cope with the demand.
Many local hotels are also offering free rooms to relatives of those involved.
The train crash is the worst Spain has experienced since a three-train accident in a tunnel in the northern Leon province in 1944.
Due to heavy censorship at the time, the exact death toll for the Torre del Bierzo disaster has never been established.
The official figure was given as 78 dead, but it is thought that as many as 250 could have been killed.
There was another serious accident in Spain in 1972 when a Madrid to Cadiz express collided head-on with a local train on the outskirts of Seville.
A total of 77 people died, with more than 100 injured.
The latest crash - one of the worst ever in Europe - comes less than two weeks after six people were killed and scores injured in a train crash just south of Paris.