Spanish police said on Monday that Abouyaaqoub is "dangerous and could be armed".
Spanish police said security operations were under way in Catalonia and on the French border as they try to find Moroccan-born Younes Abouyaaqoub, 22, who they believe is the only one of 12 suspects still at large.
Authorities were first alerted to Abouyaaqoub's presence in Subirats when a local resident saw a man who matched his description and called police.
Thirteen people were killed in the attack in Barcelona on Thursday, including seven-year-old British boy Julian Cadman.
The death toll from the attacks rose to 15 on Monday when police revealed that the fugitive stole a vehicle and killed its owner as he made his getaway from the Barcelona attack scene. His older brother Driss is among the four arrested. The responsibility for the van attack in Barcelona was reportedly claimed by Daesh outlawed in many countries, including Russian Federation.
Abouyaaqoub had been the only one of 12 accomplices still at large. He was found fatally stabbed in a Ford Focus that had forced a police checkpoint Thursday just after a van ploughed through crowds in Barcelona's Las Ramblas boulevard, killing 13 people. Of the 120 injured on Las Ramblas, nine remain in a critical condition in hospital.
The remains of two people have so far been found there.
Spain's King Felipe and Queen Letizia, Prime Minister Mariano Rajoy and Portuguese Prime Minister Antonio Costa joined Catalan leaders yesterday for a service in memory of the victims of the attack at Barcelona's towering Sagrada Familia church.
The police had fired at the auto as it forced its way through a checkpoint shortly after the Barcelona carnage, and later found Perez in the vehicle. Police also confirmed that an imam, Abdelbaki Es Satty, was among the suspects. The explosion destroyed the house, but police found remnants of over 100 butane gas tanks and materials needed for the TATP explosive, which has been used previously by Islamic State militants. "Because of that, the attack in Barcelona and the one in Cambrils were carried out in a more rudimentary way than they had initially planned".