The man, an Italian known as Giuseppe S, has been identified by Le Parisien newspaper as the main suspect in the kidnapping of 76-year-old Jacqueline Veyrac, the millionaire owner of La Réserve, an exclusive restaurant on the seafront at Nice, and the Grand Hôtel at Cannes.
Veyrac was found in the back of a white van with poorly attached false number plates two days after being snatched while getting into her SUV in a busy part of Nice.
"There was determination but a lot of amateurism," a source told Le Parisien.
Tintin and the SAS
Among the nine people detained, according to the paper, are a photographer known as Tintin, a gadget specialist who attached a tracker to Veyrac's car, and a former member of British special forces.
Investigators believe that their motive was not to collect a ransom but a personal vendetta by Giuseppe S, who apparently bore a grudge because Veyrac took control of La Réserve after he was forced to go into receivership.
This is not the first time the millionairess has been kidnapped.
She was snatched three years ago for reasons that have still not been established.
Veyrac, whose husband died five years ago, co-owns the Grand Hotel and La Reserve with one of her sons.