The manhunt continued more than 36 hours after the driver was seen fleeing the tourist hotspot on foot.
One of the suspects involved in the Barcelona attack was found dead in a auto three kilometers from where police officers tried to stop him by firing at the vehicle he was in, reports El Pais.
"Police were not touching the severely injured but were carrying the minorly wounded people off the street". They then made a decision to carry out "more rudimentary" attacks using two vehicles inspired by the attacks seen in Nice, Berlin, London and elsewhere, said Josep Lluís Trapero, head of the Mossos d'Esquadra, the Catalan regional police.
- Who are the victims?
At least 13 were killed and 125 others were injured in Thursday's atrocity.
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On Friday, emergency services in Catalonia said there were 34 different nationalities among those killed and injured in the Barcelona and Cambrils incidents.
It remained unclear how many people were involved in the terror attack.
Islamic extremist group Isis have claimed responsibility for the attacks via their Amaq News Agency. The three others were picked up in the town of Ripoll, 100 kilometres north of Barcelona. None of them had a history of terrorist activity, he said.
Moussa Oukabir's older brother, Driss Oukabir, is reported to be one of those detained.
Investigators believe the house, which was destroyed in the blast, served as a safe house for a terrorist network, and that an accidental explosion prevented the suspects from carrying out a much bigger attack.
Authorities say terrorists who attacked two cities in Spain on Thursday were apparently planning a larger, more sophisticated assault.
Officers believe that the two attacks are related to an explosion at a house in Alcanar the previous day.