Sami Chekatt, 34, was apprehended under a search warrant for terrorist conspiracy, issued after the shooting, Le Parisien newspaper reported.
The individual had reportedly been included in the list of people posing threat to France's security, the newspaper added.
The investigators are checking if he had been aware of his brother's plans and if he had been involved in providing support to the attacker, according to the outlet.
The newspaper noted that Sami Chekatt is the sixth person put in custody in the case. After the attack, the suspected gunman's parents, two other brothers, Malek et Elias, and one of the suspect's friends have also been detained.
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Late on Thursday, the police located the suspected shooter and killed him after he engaged with the law enforcement.
The Daesh* terror group reportedly claimed the responsibility for the attack.
*Daesh (ISIL/ISIS/Islamic State/IS), a terrorist group banned in Russia and a wide number of other countries.