Regional police commissioner Miquel Esquius told reporters that although investigations are continuing, police believe the driver may have dozed off or become distracted by a mobile phone or by talking to someone.
He said many of the nearly 60 students on the bus, including the 13 who died, were not wearing seat belts at the time of the early-morning crash south of Barcelona.
Seven of those killed were from Italy. The other six were two Germans, an Austrian, a French woman, a Romanian and an Uzbek.
The students were returning from a firework festival in the eastern coastal city of Valencia.
They were studying in Barcelona as part of the European Unions Erasmus exchange program.
