Francisco Garzon Amo faces multiple counts of negligent homicide for the country's worst train accident in decades. The investigation has increasingly focused on him and his failure to brake, with a witness saying he told him seconds after the crash that he was going fast and tried to stop, but couldn't.
The black box could clear up whether there was a mechanical or technical failure.
Experts will start examining the black box Tuesday under the instruction of Judge Luis Alaez. Alaez late Sunday charged Garzon, but allowed him to leave custody without bail. He was driven from the court in a police car after dark, but it was not clear where he was taken.
Several Spanish neswspapers, including leading daily El Pais, reported Monday that the driver acknowledged to the judge that the train was travelling too fast, but that he briefly stopped paying attention. A court spokeswoman said she could not comment on details of the testimony.
The spokeswoman said that before taking the driver's testimony, officials carried out checks on calls and messages made from the man's mobile phone. She was speaking on condition of anonymity in keeping with court policy.
At least one press photograph showed the man talking on a mobile phone shortly after the crash. Several news media reported Garzon told railway control in a call that he had been going too fast.
The crash has left a pall over the city of Santiago de Compostela, an important Catholic religious site that had been preparing for a feast celebrating Spain's patron saint. Shrines and regional flags with black mourning ribbons have dotted the area.
Spain's royal family and leading politicians were to attend a somber Mass on Monday in homage to the victims killed and injured. Crown heir Prince Felipe, his sister Princess Elena and Prime Minister Mariano Rajoy were among those scheduled to attend the evening ceremony in the city's ancient cathedral.
Many families of victims were also expected to attend the Mass, which is to be televised nationally. The ceremony is open to the public and a giant screen is to be set up in one of the squares outside the cathedral, where thousands of pilgrims, many with backpacks and walking sticks, traditionally gather on arriving in the city.
Officials canceled the feast of St. James of Compostela on Thursday, and turned the sports arena into a morgue.
Most of the dead were Spaniards but there were also victims from Algeria, Brazil, the Dominican Republic, France, Italy, Mexico and the United States. The train was almost certainly carrying would-be pilgrims although most usually walk to Santiago from all over Spain and abroad.
On Sunday, families of victims performed the painful task of retrieving their loved ones' belongings, collecting luggage that was being held by police.
The Spanish rail agency has said the brakes should have been applied four kilometers (2.5 miles) before the train hit the curve. The witness who rushed to the scene said in an interview broadcast Sunday that minutes after the crash Garzon told him he couldn't brake.
The resident, Evaristo Iglesias, said he and another person accompanied the blood-soaked Garzon to flat ground where other injured people were being laid out, waiting for emergency services to arrive. A photograph shows Iglesias in a pink shirt and cap helping the bloodied driver.
"He told us that he wanted to die," Iglesias told Antena 3 television. "He said he had needed to brake but couldn't," Iglesias said. He added that Garzon said "he had been going fast."
Spain's state-run train company has described Garzon as an experienced driver who knew the route well.
Officials said 70 people injured in the train accident remained hospitalized, 22 of them in critical condition.
Associated Press writers Ciaran Giles and Jorge Sainz contributed to this report from Madrid.
Copyright 2013 The Associated Press.