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STRASBOURG, France (Reuters) - A woman was put under formal investigation in France on Thursday over the murder of a four-year-old boy more than 30 years ago, reviving memories of an infamous case.

The discovery in a river of Gregory Villemin’s body in October 1984, bound hand and foot, led to a revenge killing, and the crime has remained unsolved.

A couple in their 70s was placed under investigation in the same case earlier this month.

Murielle Bolle, who was put under investigation on Thursday on suspicion of kidnapping, was 15 in 1984.

Then, she had said she saw a suspect - her brother-in-law, a cousin of the child’s father - kidnap the boy, before retracting her testimony, according to reports at the time.

Prosecutor Jean-Jacques Bosc said Bolle would be held in detention for at least four days, before magistrates decided on whether she should stay in jail.

Under French criminal law, a formal investigation means there is serious or consistent evidence pointing to likely implication of a suspect in a crime.