Four-year-old Gregory Villemin was found, his hands and feet bound, drowned in the Vologne river in the Vosges mountains of eastern France on October 16, 1984.
In one of France's most high-profile unsolved murders, the case is a saga of family jealousy and rivalries that burst back into the headlines when Jacqueline Jacob, 72, and her 71-year-old husband Marcel Jacob were arrested on Wednesday.
The couple were charged with kidnapping and an additional charge of confinement when they appeared in court in the eastern city of Dijon on Friday.
Ginette Villemin, 61, the sister-in-law of the murdered boy's father Jean-Marie Villemin, was also arrested this week before being released on Thursday.
The child's paternal grandparents were questioned in their home in the Vosges region as witnesses.
The potential breakthrough in the case came from handwriting analysis of an anonymous threatening letter sent to Gregory's father in 1983 — the year before the boy's death, the source close to the investigation said.
The analysis led to fresh suspicions about Jacqueline Jacob.
Another letter claiming responsibility for the murder that was mailed before the discovery of the body and referred to “revenge” contained “a great similarity” to the earlier letter.
Dijon prosecutor Jean-Jacques Bosc was to hold a news conference on Friday.