Train driver Francisco Jose Garzon Amo, who was hospitalized after the derailment Wednesday, was detained by police in the hospital Thursday and questioned by police Friday, the Spanish newspaper ABC reported.
Jaime Iglesias, the chief of Police of Galicia, said Garzon, 52, has been "charged with a crime related to the authorship of the accident."
The charges against Garzon include 78 counts of reckless homicide for those who were killed in the incident and "negligence," Iglesias said.
Iglesias also said the train's black box is in the hands of judicial police, but has not yet been opened.
A court "will decide how to proceed to open and the development of the information under legal protection," Igelsias said.
Judicial investigators are focusing on extreme speed on a tight curve as a possible cause of the accident in the cathedral city of Santiago de Compostela in the northwest region of Galicia.
In addition to the dead, 140 people were injured, with 95 passengers still hospitalized, 32 of them in critical condition, Spanish newspaper El Pais reported.
U.S. State Department spokeswoman Marie Harf said one American was killed in the crash and five others were injured. She said the numbers were "likely to change" but declined to elaborate.
Garzon said in a phone call immediately after the accident he was going nearly 120 mph, or more than double the 50 mph posted speed limit on the stretch of track where the train derailed, several Spanish news organizations reported.
He had taken over for another motorman 60 miles earlier, the news organizations said.
"I'm going 190 [kph] and I'm going to derail!" one of the two motormen said in a frantic call to Spanish rail company Renfe Operadora moments before the accident, Spanish newspaper ABC reported.
It was not immediately clear which driver made the call, the newspaper said.
The eight-car train, which left Madrid at 3 p.m. Wednesday, was traveling to the coast when it derailed around 8:41 p.m., about an hour before sunset.
Some train cars overturned and one was tossed up on a 15-foot embankment. At least one of two train engines caught fire.
Garzon, from Monforte de Lemos, a Galician town steeped in railway tradition, tested negative for alcohol, El Pais said.
The judge also ordered the retrieval and safekeeping of all other records vital to the investigation, including videos and reports.
A security video recorded the crash.
Spain's Ministry of Public Works opened a separate investigation into the derailment, said Prime Minister Mariano Rajoy, who visited the accident scene Thursday.
Rajoy, who was born in Santiago de Compostela, said in a statement he could "only express my deepest sympathy as a Spaniard and a Galician."
He declared three days of official mourning.
King Juan Carlos and his wife, Sofia, also rushed to Santiago de Compostela.
Garzon had expressed a love of speed on Facebook March 8, 2012, posting a photo of a locomotive speedometer needle stuck at 200 kph, or about 125 mph, ABC reported, showing the photo and comments from the Facebook page before they were removed late Thursday morning.
The comments from Facebook friends indicated astonishment.
"Dude, you're going full speed, braaaaake," one person wrote.
"Christ, you're doing 200km/h," another said.