The train driver in last week’s crash in Spain was talking on the phone to train company officials when it derailed, according to the BBC.
The train was travelling at 153km/h (95mph) at the time, investigators at the Court of Justice of Galicia added.
Crash investigators had opened the train’s “black-box” data recorder to find the cause of the crash, which left 79 people dead
Investigators say the brakes were activated shortly before the crash.
The speed limit on the sharp bend where the train derailed was set at 80km/h (49mph).
“Minutes before the train came off the tracks he received a call on his work phone to get indications on the route he had to take to get to Ferrol. From the content of the conversation and background noise it seems that the driver consulted a map or paper document,” a court statement said.
Mr Garzon is suspected of reckless homicide and was released from custody in Santiago de Compostela, where the crash occurred, on Sunday but remains under court supervision.
He must appear before a court once a week and was not allowed to leave Spain without permission.
His passport has been surrendered to the judge and his licence to drive a train has been suspended.
Under Spanish law, his legal status is that he is suspected of being involved in 79 counts of reckless homicide.
Officials said he had admitted negligence by being careless when rounding a bend too fast.
All eight carriages of the train careered off the tracks into a concrete wall as they sped around the curve on the express route between Madrid and the port city of Ferrol on the Galician coast.
On Monday, a mass was held in the cathedral in Santiago de Compostela.
Prime Minister Mariano Rajoy, along with the heir to the Spanish crown, Prince Felipe, and his wife Princess Letizia, joined the grieving families and local residents in the cathedral as the city’s archbishop prayed for the dead.