Police had earlier said they were searching for Oukabir and three other men. Now only one remains unaccounted for. Four suspects have been arrested.
Thirteen people were killed when a van, said to be rented by Oukabir, rammed crowds in Barcelona's Ramblas district.
Police say the suspects had been planning more sophisticated attacks.
They said blasts on Wednesday in Alcanar, a town south of Barcelona, deprived plotters of bomb material so they carried out simpler attacks using vehicles.
Moussa Oukabir, thought to be 17, was a Spanish citizen from the northern Catalan city of Girona. Most of the other suspects were Moroccan.
He is suspected of using his brother's documents to rent the van used in the Barcelona attack and another found hours later in the town of Vic, north of Barcelona, that was intended as a getaway vehicle.