Train driver Francisco Jose Garzon Amo is officially held in the hospital where he is recovering after 78 people were killed in a rail crash in north west Spain.
The eight-carriage train came off the tracks, hit a wall and caught fire just outside the pilgrimage destination Santiago de Compostela in north western Spain on Wednesday night.
Galician police chief Jaime Iglesias said Mr Garzon Amo would be questioned “as a suspect for a crime linked to the cause of the accident”.
He said the driver is being guarded by police and cannot yet testify because of his medical condition.
Mr Iglesias said he did not have details of the medical condition but that it could delay the driver’s statement.
Meanwhile investigators have taken possession of the “black boxes”.
A second probe launched into the catastrophe will look at the train’s in-built speed regulation systems.
Early indications suggest the train reached 118mph – more than twice the 50mph speed limit – when it crashed while heading into a curve.
Video footage from a security camera showed the train, with 247 people on board, hurtling into a concrete wall at the side of the track as carriages jack-knifed and the engine overturned.
One local official described the aftermath of the crash as like a “scene from hell“, with bodies strewn next to the tracks.
Visiting the hospital in Santiago where most of the wounded are being treated, King Juan Carlos said: “All I can say is that all Spaniards are united in this moment and we are with the victims, with the families and with the friends of the victims.”
One Briton has been confirmed by the Foreign Office to be among 168 injured passengers, while the regional government in Galicia said it had so far only been able to confirm the identities of 67 bodies.
The authorities were still working to identify the other 13.
DNA tests are expected to be carried out on those with catastrophic injuries to identify them, with results available in the coming days.
Meanwhile, more than 30 people including four children remain in a critical condition, according to reports.
Many of those on the train would have been pilgrims travelling to Santiago’s St James’ Day festivities, which celebrate the disciple of Jesus whose remains are said to rest in a shrine.
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.
As the country mourned, Spanish media reported Mr Garzon Amo allegedly posted on Facebook about how fast he was driving a train in March last year.
The 52-year-old, who together with a second driver survived the crash and is being treated in hospital for minor injuries, allegedly posted a picture of a train speedometer at 200kph (pictured above). 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 allegedly wrote.
Mr Garzon Amo is a 30-year employee of Renfe who became an assistant driver in 2000 and a fully qualified driver in 2003.
He is believed to have taken control of the train from a second driver about 65 miles 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 190kmh, I’m going to derail.”
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.”
With the dead being taken to a makeshift morgue set up in a Santiago indoor sports arena, Spanish prime minister Mariano Rajoy, who was born in the city, visited the crash scene and declared three days of national mourning.
He said judicial authorities and Spain’s Public Works Ministry had launched parallel investigations into what caused the crash.