Prosecutors in eastern France on Thursday charged an ex-soldier with murdering a nine-year-old girl who went missing from a wedding party in the French Alps in August.

Nordahl Lelandais, 34, had already been charged with kidnapping Maelys de Araujo, and has been in custody since September 3. He denies the charges.

He was charged after nearly eight hours of questioning on Thursday, during which he answered "calmly, and kept his cool," Grenoble prosecutor Jean-Yves Coquillat told a news conference.

Coquillat said enhanced video footage showed a "small, frail form in a white dress" in the passenger seat of Lelandais's car as he drove away from the wedding in the early hours of August 27, which was not present in footage of his return to the party.

Maelys was wearing a white party dress when she disappeared from the wedding she was attending with her parents in the village of Pont-de-Beauvoisin, near the town of Chambery in France's tranquil Isere region.

The prosecutor said Lelandais put his phone in airplane mode at around the same time partygoers realised Maelys was missing, in a suspected bid to mask his whereabouts.

Despite an exhaustive search and hundreds of interviews, police have failed to find any trace of the girl in a case that has gripped the nation.

Maelys's ninth birthday was November 5.

Her DNA was found in Lelandais's car.

The former military dog trainer told investigators that Maelys briefly got into his car with another child to see if his dogs were there, but he claims she later returned to the wedding party.

He had arrived late for the wedding, and left the party three times, including at around the same time as Maelys disappeared, investigators said.

He also cleaned his car thoroughly the day after the wedding, which police considered suspicious. He said he was preparing to sell it.