Police investigating a 1987 murder that mystified France believe they have identified the victim, a four-year-old girl, and have arrested her parents on suspicion of killing her.

The girl’s body, bearing signs of horrific abuse including burns from an iron and human bite marks, was found in a ditch alongside the A10 motorway in central France in August 1987.

Investigators at the time said they were dealing with a suspected case of cannibalism.

The murder sparked what was at the time France’s biggest investigation. An alert was sent to more than 30 countries, but in 1997 the case was declared unsolved.

The trail remained cold until the arrest of a man in 2016 for assault led to a DNA match – he was the dead girl’s brother. Police then tracked down and arrested the parents, prosecutors in the central city of Blois said.

The girl, in the absence of a name, was known as “the little martyr of the A10”. Posters appealing for information noted that she was 95cm (3ft) tall with curly brown hair and brown eyes.

Police visited 65,000 schools as part of the effort to identify her, speaking to 6,000 doctors and school assistants in the hope of tracking down her family.

When her brother’s DNA was entered into a national database, investigators were alerted to a match with material found on her clothes and the blanket she was wrapped in. This allowed them to track down the parents, a couple of Moroccan origin now in their sixties.

Records showed they had stopped claiming family welfare payouts for one of their seven children, and a source close to the investigation said the mother had claimed the child was in Morocco.

The parents, who separated in 2010, were detained on Tuesday in the central city of Orleans on suspicion of murder, child abuse and preventing the lawful burial of a body. They were due to appear before a judge on Thursday.

A source close to the investigation said the girl’s father was claiming she died at the hands of the mother.

Etienne Daures, the prosecutor in the original case, said he hoped “to be able to give a name” to the girl, who was buried in a cemetery in the town of Suevres near where her body was found.

Even now, people regularly replace the flowers on her grave, where the unnamed tombstone reads: “Here rests an angel.”