The driver of the train that derailed killing 78 people has refused to answer initial questions, Spanish police say.
Francisco Jose Garzon Amo - hurt in Wednesday's crash - is under guard in hospital.
He is suspected of driving too fast round a bend. Reports say the train was travelling at more than double the speed limit at the time of the crash.
The case should now "proceed to a judicial process", the police added. Spain is in three days of mourning.
The police chief in the Galicia region, Jaime Iglesias, said earlier that the driver would be questioned "as a suspect for a crime linked to the cause of the accident".
No date has been fixed for his appearance before the judge, a spokeswoman for Galicia's High Court, which is leading the investigation, told the AFP news agency.
Police also put the confirmed number of deaths at 78 - down from 80 announced earlier.
They said the difference arose because human remains had been wrongly identified in the initial stages.
At least 130 people were taken to hospital after the accident near the north-western city of Santiago de Compostela, and 95 are still being treated.
The 32 seriously injured include children. People from several nationalities are among the wounded, including five Americans and one Briton. One American was among the dead.