A months-long investigation into the mysterious death of a French toddler took a surprise turn on Tuesday when police arrested four people including the boy's grandparents on suspicion of murder and concealment of a body.

The death of Emile Soleil, a boy of two-and-a-half who went missing in a French Alpine village in July 2023, had remained unexplained even after some of his remains were discovered nine months later.

Prosecutors at the time said the cause of his death could have been "a fall, manslaughter or murder".

Emile had been staying at the summer home of his maternal grandparents in the tiny hamlet of Le Haut-Vernet when he vanished. Emile's mother and father were absent on the day he disappeared.

In a sharp turn of events on Tuesday, the boy's 59-year-old grandfather, Philippe Vedovini, and his wife were arrested on suspicion of voluntary homicide and concealment of a corpse, said Aix-en-Provence public prosecutor Jean-Luc Blachon.

Two adult children of the couple were also arrested, Blachon added.

Tuesday's arrests were the result of fact-finding "over recent months", the prosecutor told reporters.

Earlier this month investigators had returned to the village, sparking talk of a development in the case.

A large flowerpot with traces of blood had been seized by the investigators in front of a local church, a source close to the case told AFP.

Several years ago the boy's grandfather had been questioned over alleged violence and sexual assault at a private school in the 1990s where he was a scout leader.

Police had initially considered his possible involvement in Emile's death as only one of many hypotheses.

"Perhaps what is happening is not what we had expected," said Philippe Vedovini's lawyer, Isabelle Colombani, when questioned by police in Marseille.

But she added that "being placed in police custody means nothing."

Julien Pinelli, who represents Emile's grandmother, Anne Vedovini, said that she wanted nothing more than to learn "the truth about this tragedy."

"And if this measure is necessary to find the truth, of course she will submit to it," he said.

In the southern village of La Bouilladisse, an AFP reporter saw police conduct a search of the couple's main home, an affluent farmhouse. They have seized an SUV and a horse trailer.

One of the neighbours, who did not wish to give his name, saw the police detain Vedovini, who is an osteopath, and his wife at 7 am in the morning.

"For me, they're quiet neighbours," he said. "They've been under pressure for the past two years, that's for sure. They have lived in a reclusive manner."

Emile was last seen walking alone on a street in Le Vernet, 1,200 metres (4,000 feet) up in the French Alps, in the late afternoon of July 8, 2023. He was wearing a yellow T-shirt, white shorts and tiny hiking shoes.

A massive search failed to find any sign of the boy.

Nine months later, a walker discovered his skull and teeth 1.7 kilometres from the village.

Police later found more bones and items of the boy's clothing.

In February, a traditional Catholic funeral mass for the toddler was held in the Basilica of Sainte-Marie-Madeleine, a Unesco-listed medieval Gothic cathedral said to house relics of Mary Magdalene, a follower of Jesus Christ.

Several hundred mourners were in attendance.

Emile's grandparents and parents are devout Catholics.

Within hours of the ceremony, the grandparents published a statement saying "the period of silence must yield to the period of truth."

"We need to understand. We need to know," they said. (AFP)