Jacqueline Veyrac, 76, the owner of the Michelin-starred La Reserve restaurant in the French Riviera city of Nice was snatched last October as she was getting into her SUV.
She was discovered 48 hours later after being spotted by a passerby bound, gagged and tied to the van's floor.
The legal development comes after five people, four men and a woman, were questioned by investigators about their possible participation in the kidnapping after being arrested earlier this week.
In total, 16 people have already been implicated in the case, including nine who have been imprisoned.
The former manager of a gourmet restaurant in Nice, identified as Giuseppe S, is suspected of ordering the kidnapping because he harboured a grudge against Veyrac.
Originally from Turin, he managed La Reserve from 2007 until 2009 when his company went into liquidation.
Investigators believe this caused him to resent Veyrac, and suspect the kidnapping was a bid to recover money lost during the liquidation by demanding a ransom from those close to her.
Veyrac, whose husband died five years ago, co-owns the five-star Grand Hotel, as well as La Reserve, with one of her sons.
Veyrac was targeted in another attempted kidnapping in 2013, commissioned by the same man, according to the investigation.