Spanish police say three Moroccan nationals and one Spaniard have been in arrested in connection with suspected militant attacks on August 17 that left 14 people dead.
Police in the Catalonia region also said the driver of a van who rammed into crowds in central Barcelona could be among five suspects killed by police after a separate attack in the nearby town of Cambrils.
The head of Catalonia's police told an August 18 news conference, however, that there were "clues" but no "concrete proof" of this.
Thirteen people died in the Barcelona attack, while one woman died on August 18 from injuries sustained when the Cambrils assailants drove a car into people, injuring several passersby.
Police said the five Cambrils assailants shot by police were wearing fake suicide vests, and that the incident was connected to the van attack in Barcelona.
The arrested suspects are between the ages of 21 and 34, police said.
Thousands of people, among them King Felipe VI and Prime Minister Mariano Rajoy, held a minute of silence on August 18 at noon in Barcelona's main square for attack victims.
The extremist group Islamic State claimed responsibility for the attack on Barcelona's most famous avenue, Las Ramblas, which was packed with tourists. The death toll could rise, with more than 100 injured, authorities said.
The injured and dead came from 24 countries, authorities said on August 18.