The man, in his early 30s, was detained Tuesday in the southwestern town of Albi on suspicion of links to Moroccan Driss Oukabir, alleged to have rented the van used in the Barcelona rampage.
A total of 16 people were killed when a van drove into crowds on the popular Las Ramblas boulevard in the heart of Barcelona and in a knife attack in the nearby resort of Cambrils last August.
The source said investigations would determine the nature of links the suspect had with Oukabir, who was arrested shortly after attacks which were claimed by the Islamic State group.
Two other two men who were detained in France on Tuesday have been released for lack of evidence, the source said.
The arrests were made during a joint operation with French and Spanish police in the south and southwest of France.
The identity papers of Oukabir were allegedly used to hire the van used in the Barcelona attack and driven by Younes Abouyaaqoub, who was shot dead by police along with four accomplices after an attack the following day in the coastal town of Cambrils.
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