You are correct in surmising that the accident investigation procedure in Spain is quite different from that of the UK of USA. Accident investigations are undertaken by a judge who calls in expert witnesses to assist him or her in the case. The sole purpose of this judicial investigation is to allocate blame.
It is unusual in such inestigations in Spain to take human factors into account. During the long investigation of the Valencia Metro crash in which 43 people died in 2006, the judge determined that the crash had been caused by the excess speed of the train. The investigation was re-opened twice by the relatives of the deceased. At no time was the reason why the driver might have been driving at excess speed ever investigated.