A couple in their 70s and a woman have been detained for questioning by police, 32 years after a crime that horrified France.
Grégory Villemin was four when his body was found tied up in the Vologne river in eastern France.
His killer was never found although relatives were wrongly sent to jail.
The murder has continued to haunt France ever since, but advances in DNA technology have helped police shed further light on the case.
Years of mistakes
It was on 16 October 1984 that Grégory Villemin's body was found and a magistrate opened what became one of the biggest mysteries in French criminal history.
Hours before the boy was discovered, his hands and feet tied, his uncle said he had received a phone call from someone claiming to have kidnapped him. The following day, the child's parents received a letter that said: "Your son is dead, I have been avenged."Image copyright AFP Image caption Christine Villemin (third from L) was for years wrongly blamed for the murder
The next month, a cousin of the boy's father, Bernard Laroche, was arrested when his sister-in-law testified against him. He was released in 1985 when she retracted her statement, but shot dead by Jean-Marie Villemin, the boy's father, weeks later.
The father went to jail for Laroche's murder for several years. The boy's mother, Christine Villemin, was also jailed for her son's murder but later cleared. Both were later given compensation by the state for miscarriage of justice.
The anonymous phone-calls and poison-pen letters continued over the years, and the case was reopened first in 2000 and then again in 2008 in an attempt to identify the DNA on the letters. Three separate traces of DNA were found but not identified.
Three in police custodyImage copyright AFP Image caption Both of the boy's parents spent years in jail and later received compensation for a miscarriage of justice
Little news has been heard in the "Grégory affair" since police said in 2013 that DNA tests had brought the investigation no further.
That was until about 08:00 (06:00 GMT) on Wednesday, when investigators from Dijon detained a couple in their 70s in the village of Bruyères, in the Vologne river valley.
They were placed in police custody on suspicion of complicity in the murder, failing to report a crime or helping a person at risk, according to L'Est Républicain website.
A third person described as close to the Villemin family was also being held, and a fourth, reportedly the victim's grandmother, was taken in as a witness, French media reported.