DIJON, France (Reuters) - A couple in their 70s were put under investigation in France on Friday over the grisly murder of their four-year-old great nephew over 30 years ago, reviving memories of an infamous case and sweeping news of an impending election off the front pages.

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The discovery in a river of Gregory Villemin’s body in October 1984, bound hand and foot, led to a revenge killing, and the murder has remained unsolved even though it has been reopened several times.

Now the arrest of Jacqueline and Marcel Jacob, following forensic improvements in criminal investigation, has gripped the nation once more and is dominating newspapers and TV news bulletins even as France prepares for a final round of elections on Sunday.

Gregory’s body was found in the Vologne river near his home in a remote village in eastern France.

The “Gregory Affair” case was reopened most recently in 2008, when French detectives found DNA traces on a letter, raising hopes that advances in DNA identification might help identify the killer at last.

The letter in question was one of a series of anonymous and threatening ones sent to Gregory’s parents before and after Gregory’s death, claiming to be from the killer.

Jean-Jacques Bosc, prosecutor in Dijon, said that thanks to improved methods in the study of handwriting in conjunction with other evidence, the couple - aunt and uncle to Gregory’s father - were now under investigation for “kidnapping leading to death.”

“The people who took part in that kidnapping are the authors of the crime,” Bosc told reporters at a news conference on Friday, adding that investigators believed the original suspect in the case, their nephew Bernard Laroche, now dead, was also involved.

Being put under formal investigation in France can be a step on the way to charges, but it does not necessarily lead to a trial. The couple’s lawyers and a lawyer for Laroche’s widow said their clients denied any involvement, and that they would contest the investigation.

Laroche was charged with the murder of Gregory soon after it took place, but he was later freed after evidence against him was thrown out by prosecutors.

Convinced that his cousin Laroche was the murderer, Gregory’s father Jean-Marie Villemin shot him dead in 1985 and served four years in jail for the act.

Gregory’s mother, Christine Villemin also became a suspect because witnesses said they had seen her at the post office on the day that one of the mysterious letters was sent.

She was placed under formal investigation but cleared in 1993. The case was reopened in 1999, and again in 2008.