Officials believe a 12-member cell was responsible for the vehicle attacks in Barcelona and the seaside town of Cambrils, which left 14 dead and around 130 injured.
Three of the group remain unaccounted for by police, but it is believed two of those were killed in an explosion on Wednesday at the house in Alcanar where the terror plot was hatched.
Police declined to confirm that Younes Abouyaaqoub, a 22-year-old Moroccan, was the terrorist who drove the van down Las Ramblas promenade on Thursday.
Spanish police official Josep Lluis Trapero is said to have told a press conference there is no specific information to indicate the missing suspect's whereabouts, but some reports suggest he may have crossed the border into France on foot.
All of the main suspects are believed to have lived in the north-eastern town of Ripoll.
On Sunday, the family of a seven-year-old boy killed in the Barcelona attack said they were blessed to have had him in their lives.
Julian Cadman, a dual British-Australian national, was separated from his mother on Las Ramblas during the massacre, prompting relatives to launch an online appeal for information.
Confirming his death, his family said: "As he was enjoying the sights of Barcelona with his mother, Julian was sadly taken from us.
"He was so energetic, funny and cheeky, always bringing a smile to our faces.
"We are so blessed to have had him in our lives and will remember his smiles and hold his memory dear to our hearts."
Officials believe the terror group behind the attacks were plotting much deadlier carnage using explosives favoured by Islamic State militants, the Associated Press has reported.
Three vehicles were rented using the credit card of Abouyaaqoub and police believe the cell wanted to load the vans with explosives for a big attack, but were forced to change their plans after the house in Alcanar blew up.
The group is said to have stored more than 100 gas tanks and explosive ingredients at the house.
Five members of the cell were shot dead, four are in custody and as many as two were killed in the explosion, an interior minister said.
Some 34 nationalities were among those wounded in the attacks in Las Ramblas and Cambrils, which lies around 70 miles to the south west of Barcelona.
Victims of the atrocity in Barcelona have been identified as British/Australian, Italian, Portuguese, Belgian, Spanish, Spanish/Argentine, Canadian and American.
The victim of the second assault in Cambrils in the early hours of Friday - which brought the death toll to 14 - was identified as a Spanish woman.