The driver of the high-speed train, Francisco José Garzón, had been travelling well above the speed limit when the accident occurred, said investigators from Spain's railway accident commission (CAIF) in a new 266-page report.
He had also been distracted by a call from the train's conductor, the report found.
Investigators said the high-speed train bound for the pilgrimage city of Santiago de Compostela had been travelling at 190kph (118mph) when it entered a curve with a speed limit of just 80kph.
The train crashed into a concrete embankment with four carriages overturning in the resulting smash. One was torn apart by the force of the collision, in an accident which captured world headlines.
The driver had not failed to apply the brakes to slow the train before entering the so-called Angrois curve and had not actually been driving in the 6km before the dangerous stretch of track, the CAIF said in its report.
Garzón's decision to answer a call from the train's conductor was a contributing factor in this failure to brake, the investigators concluded.
Garzón is currently facing 79 counts of negligent homicide for his role in what was Spain's worst train accident in 40 years.
While the CAIF laid the blame for the July 24th 2013 accident with the train's driver, their report also contains a swathe of recommendations on how to improve the safety of the rail network.
These include improved management of the installation of automatic braking systems to slow trains travelling too fast and new onboard communications systems so that train drivers don't need to receive mobile phone calls while working.