Francisco Jose Garzon Amo was at the controls when the train rounded a curve and flew off the tracks. The train operator was talking to fellow workers at the state-owned Renfe railroad company, the BBC reported Tuesday.
Garzon is suspected of reckless homicide in the deaths but prosecutors had yet to officially charge him.
Garzon was released from a Spanish jail Sunday. He remained under court-ordered supervision.
The train's "black box" data recorder has been opened and investigators are sifting through the information that should give them a more concrete explanation for what caused the deadly crash.