The Moroccan-born 22-year-old is accused of carrying out Thursday's van rampage on Las Ramblas, which killed 13 people.
But Catalonia's police chief today said Abouyaaqoub could have crossed the border between Spain and France following the ISIS attack.
Josep Lluis Trapero told a press conference this afternoon: "We don't have any specific information on this but it cannot be ruled out.Related articles
If we knew that he was in Spain and where, we would go after him
Josep Lluis Trapero
"If we knew that he was in Spain and where, we would go after him. We don't know where he is."
Mr Trapero insisted that border checks had been reinforced immediately after the tragedy.
Mossos d'Esquadra, Catalan's police force, tweeted this morning: "Security checks continue in Catalonia and especially near the border with France.
"We appreciate the understanding of citizens."