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.
The case of "Little Gregory" is one of France's most high-profile unsolved murders, a saga of family jealousy and rivalries that burst back into the headlines when Jacqueline Jacob, 72, and her 71-year-old husband Marcel were arrested on Wednesday.
They were both remanded in custody.
Both have denied any involvement, Dijon prosecutor Jean- Jacques Bosc later told a news conference.
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For the moment they do not have an alibi that is "either confirmed or backed up", Bosc said.
Explaining the nature of the charges, he said Gregory would not have died unless he had first been kidnapped.
"Gregory was kidnapped from his parents' home and held for a certain time before his death," the prosecutor said.
Handwriting analysis of a letter sent to Gregory's father in 1983, a year before the boy was killed, led to fresh suspicions about Jacqueline Jacob.
Another letter claiming responsibility for the murder, which was mailed before the discovery of the body and referred to "revenge", was submitted to linguistic analysis that compared the word choices with those in the earlier missive.
The lawyer for Marcel Jacob protested today that he should not have been charged because there was no new evidence against the couple.
Ginette Villemin, 61, the sister-in-law of the murdered boy's father Jean-Marie Villemin, was also arrested this week but was released on Thursday.
The child's paternal grandparents were questioned in their home in the Vosges region as witnesses.
The family was from a modest background and investigators have long believed that the motive for the murder was jealousy towards the boy's father, Jean-Marie Villemin, who had just been promoted to foreman at his factory, and his attractive wife Christine.
A photograph of the smiling face of Gregory with his tousled hair still strikes a powerful emotional chord in France.
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