They are hunting for Moroccan-born Younes Abouyaaqoub, now named by Spanish media as the suspected driver.
A man previously reported as the key suspect, Moussa Oukabir, 17, was one of five men killed by police after a later attack in Cambrils, west of Barcelona.
Police say the suspects had been planning more sophisticated attacks.
They said blasts on Wednesday at a house in the town of Alcanar deprived plotters of bomb material, so they carried out simpler attacks using vehicles.
Abouyaaqoub, 22, lived in the town of Ripoll to the north of Barcelona. Three people have been arrested in Ripoll and one person in Alcanar.
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Oukabir had previously been seen as the main suspect - but late on Friday police chief Josep Trapero told local TV that the theory that he was the driver now "had less weight".
El Pais newspaper said there was a growing belief that Abouyaaqoub was the main suspect.
Oukabir 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.
In the early hours of Friday police shot dead five attackers, including Oukabir, in Cambrils after the men drove a car into pedestrians, killing one woman and injuring six other people.
The attackers' vehicle overturned and when the men got out they were quickly fired upon by police. One was reportedly brandishing a knife.
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