Between 1985 and 1990 Lyons and surrounding areas in central France were terrorised by a gang of armed robbers who wore carnival-mask disguises and killed in cold blood.
Today, more than five years after the gang's last stand, the 14 men believed to be responsible go on trial in the city.
The most remarkable feature of the case is not its scale, nor even the bizarre detail of the masks, but the fact that five of those in the dock are former members of the Lyons police force, whose colleagues had long been blamed for failing to solve the wave of serious crime in their precinct. The five were all assigned to a run-down area near the main railway station, where the police-station chief was an acknowledged depressive and alcoholic (he subsequently committed suicide), and officers whiled away the hours drinking whisky and playing poker.
In those five years the area's crime rate rose by almost 70 per cent, the number of charges laid fell by more than a third and Lyons district III became a standing joke in the force.
The lawyer for one of the accused said there was "a total loss of discipline". Jean Giovanetti, 49, regarded as the "brains" of the gang, is described as a figure of considerable charm and a natural leader. A one-time medical student and successful hotel manager, he is said to have been bored and increasingly unhappy with the difference between his own police lifestyle and that of the criminals he - occasionally - questioned.
The "hard man" of the gang is said to have been Michel Lemercier, 45, who was, allegedly, already known for taking bribes to release petty offenders.
Temptation is reported to have been put in the policemen's way by a petty criminal who told them that bars which doubled as betting offices had no additional security and robbing them was "a piece of cake". From bar/betting offices, the gang graduated to supermarkets, then to banks. They committed their first murder when they shot a bar customer who tried to intervene. In January 1989 they shot two security guards in the car park of a supermarket.
One of Giovanetti's police tasks was to control the progress of investigations. The case was solved by what seemed to be pure chance. In November 1990, police on a routine inspection caught a garage mechanic fixing a false number-plate to a stolen car. The garage was put under surveillance and the (regular) police learnt of a building-society raid planned for 12 November 1990. The robbers were caught red-handed.
Although these events are more than five years old, the trial turns the spotlight on the state of the French police at an awkward time. Memories are fresh of the in-fighting and bungling associated with the anti-terrorism investigation over the summer, and the televised shooting, coincidentally also near Lyons, of the Algerian-born terrorist suspect Khaled Kelkal.
This may be the reason why the slapstick "cops and robbers" aspect of the Lyons case has been consistently overlaid in recent French reports with a mixture of embarrassed shame and deep moral outrage. The guilt of the five policemen is not being doubted; what fascinates now is why they did it.