A man has been arrested for alleged links to the extremist cell in the attacks in Barcelona and a nearby town, Spain's interior ministry said.
The 24-year-old suspect of Moroccan origin purchased materials for explosives in his name and he provided vehicles for the cell members, the ministry said.
The suspect is the fifth person to be arrested in the investigation.
Three, including the latest suspect, are in custody and two others have been freed but remain under investigation.
The Islamic State group claimed responsibility for the van attack in Barcelona's Las Ramblas district and the vehicle and knife attack in the coastal town of Cambrils that killed 16 people on August 17 to 18.
Eight members of the extremist cell died or were shot dead by police.