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.
The day after Gregory's body was found, a letter arrived at the home of the child's parents claiming responsibility for the murder and calling it “revenge”. The couple had been receiving anonymous hate mail since 1981.
Investigators took that letter seriously because it seemed to have been posted before the body was found.
Handwriting experts identified Jacob — the uncle of Gregory's father who was among those held on Wednesday — as possibly being the writer of the letter.
Bernard Laroche, a cousin of the child's father, was charged with the murder a month after the boy's death, based on evidence given by a teenage sister-in-law.
He was released after she withdrew her claims, only to be shot dead in March 1985 by Gregory's father Jean-Marie Villemin who spent two and a half years in prison for the crime.
The dead boy's mother, Christine Villemin, was herself charged with the murder in 1985. But she was finally cleared eight years later and all charges against her were dropped.
The arrests on Wednesday were on charges of being an accomplice to murder, failing to denounce a crime and failing to help someone in danger, said local newspaper l'Est Republicain, which broke the story.
In the course of this latest probe into the case, investigators questioned around a hundred witnesses, some for the first time, prosecutor Bosc said in his statement.
They also analysed more than 2,000 anonymous letters received by those involved in the affair, including some to magistrates working on the case.