A major manhunt is still under way for the driver of the white Fiat van used to mow down pedestrians in the Barcelona terror attack.
Moussa Oukabir, 18, been named as a suspect in Spanish media.
Four individuals have so far been arrested in connection with the rampage, in which 13 people were killed and more than 100 others injured.
There are reports that police believe two vans were used - one for the attack and a second as a getaway vehicle.
One of the men arrested was Oukabir's older brother Driss.
The 28-year-old reportedly handed himself into authorities in Spain, but denied involvement in the attack, claiming his younger brother stole his documents and used them to hire the van.
Another of the detained individuals is a Spanish national from Melilla.
A third suspect was arrested later, Catalan authorities said, without giving information about the detainee's nationality.
The arrests were made in the northern Catalan town of Ripoll and in Alcanar, the site of an explosion on Wednesday night which killed one person.
Spanish authorities believe that blast is linked to the van attack.
So-called Islamic State has claimed responsibility for the attack, the deadliest on Spanish soil since more than 190 people died in the Madrid train bombs in 2004.