When their bodies were discovered in scrubland more than two months later, it appeared that they had been executed with a hunting rifle.
Now, after 10 years of false trails, police have arrested Cedric Mabille, who would have been aged only 17 at the time of the killings.
Mr Mabille, a criminology student at Montpellier University who is said to be fascinated with firearms, has allegedly confessed to the murders of Gilles Naudet, a bank clerk aged 25, and his girlfriend Anne-Sophie Vandamme, a social worker aged 24 - known as the 'fiances de Fontainebleau'.
Police are also questioning Mr Mabille's father, a former pilot living at Boulogne-sur-Mer, and a friend, as possible accomplices to the apparently motiveless crime.
After Naudet and Vandamme disappeared on October 31 1988, hundreds of police searched Fontainebleau's rock formations and reed marshes. A medium was even called in.
Then on January 10 1989 their bodies were found by a forest guard. The couple and their dog, Dundee, had been shot dead.
The disappearances scared away many walkers who had used the area, and over the years there have been many reports that terrorists and gangs of violent poachers were using the forest near Napoleon's favourite chateau.
A suspected serial killer and members of rightwing paramilitary groups were among those questioned during the 10-year inquiry. Fifteen people were held in custody before being cleared.
The inquiry team was led by Dominique Bellanger, a detective sergeant whose team spent 200,000 hours interrogating 1,500 suspects and tested 1,600 guns. 'We just couldn't get this murder out of our minds,' he said 'It became an obsession.'
Hopes of solving the crime were fading until last month, when the police rounded up 13 poachers, whose testimony led them to Mr Mabille, aged 28.
One witness told police that three years ago Mr Mabille, wearing battle dress and carrying a hunting knife, had attacked him for 'hunting on his private ground' before being overpowered by a karate expert. The fight was reported to the police but it failed to throw up any link with the case of the fiances.
Pierre Coly, the mayor of Gironville, near Fontainebleau, where Mr Mabille grew up, confirmed that the suspect was fascinated with guns and camouflage. Before moving to Montpellier, he had studied weapon making in Belgium.
One of the more lurid theories is that the killer may have hunted the couple like prey. Jacqueline Vandamme, the mother of one of the victims, said: 'How can people like that go on calmly living? Our children seem to have been murdered for nothing.'
