A total of 14 people were killed and more than 100 injured after a van plowed into crowds of people in Las Ramblas.
One of those being sought by police was Moussa Oukabir, a police source said, but it was unclear what his suspected role may be.
A French police source has said they are looking for the driver of a white Renault Kangoo van that may have been used by those involved in the Barcelona attack.
A woman cries in front of a memorial tribute to the victims of the terrorist attack in Barcelona, Spain, Saturday Aug. 19, 2017. He added that the terror threat level would remain at four, meaning the threat is "high", rather than increasing it to level five, which means an attack is believed to be imminent.
The seven-year-old boy, Julian Alessandro Cadman, was separated from his mother when a van rammed into a crowd of people in the pedestrian area in central Barcelona on Thursday, reports said, citing a post on social media from the boy's grandfather.
Police are still seeking the driver in the Barcelona attack, for which ISIS claimed responsibility, according to the SITE Intelligence Group.
Catalan police have identified the five terrorists who were shot dead in Cambrils.
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On Thursday, Prime Minister Justin Trudeau condemned the attack in Barcelona while expressing condolences to the victims' friends and families.
An unnamed Spanish national was killed in the explosion with another person inside the house is seriously injured. "And we aren't afraid because that's what they want, the Islamic State, they want to scare us so that we stay at home".
One witness to the Cambrils attack said he saw a man running down the road shouting something. In all, four suspects have been detained following the twin vehicle attacks, one down a busy Barcelona street, the second, 75 miles south in the beach town of Cambrils.
No suspected motive for that attack was given.
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.
Police say the two incidents may have been linked to an incident in another Spanish city on Wednesday.