The sun was shining over the Alps when Sghair Lamiri, a convicted robber, took an afternoon stroll with a friend in the yard of a remand centre near Grenoble.

Without warning, the short and stocky 29-year-old boxing enthusiast fell to the ground, a bullet in his head – picked off by a distant sniper. His friend was wounded in the hand by further rounds from the invisible gunman.

The ruthless execution has embarrassed the French prison authorities, who had never lost an inmate to an outside killer. It also took to a new level a vendetta in which ten young men have been murdered in two years in a war for control of the drug trade in Grenoble and nearby ski resorts.

Lamiri had been sentenced to eight years for armed robbery but had been moved to the remand prison to be questioned about the tit-for-tat murders. These began in 2003 after his elder brother, Lassad, a powerful drug dealer, was shot dead at the age of 27.

Two gangs, known as Les Gitans (Gypsies) and Les Maghrébins (North Africans), based in Grenoble housing estates, have adopted ultra-violent methods that contrast with the codes of the old-school Corsican and Italian mafiosi they displaced. Police are battling against a similar underworld shift around Paris, Marseilles and other big cities.

Officers managed to arrest Lamiri’s alleged shooter as he tried to make his getaway from a wooded hillside overlooking the Varces prison after he had fired six rounds from a distance of 300 yards – but they were lucky. Gendarmes had spotted a parked motorcycle with the numberplate of a stolen car and had decided to wait for the driver to return. He turned out to be a 58-year-old convicted robber who was carrying a 7.65mm rifle with a telescopic sight.

Rachida Dati, the Justice Minister, said from the scene that the murder was a blow to the prison system but congratulated the gendarmes on their catch. “The man denies the facts but he was arrested when he was getting to his motorbike, which had a false licence plate and his rifle with telescopic sight was still warm,” she said.

About 100 police and firefighters were sent to the jail to quash a riot and put out fires started by prisoners, most of whom are awaiting trial.

France has taken elaborate precautions over the past 20 years to stem a series of prison breakouts via helicopter but the authorities had ignored warnings that the holding centre at Varces was vulnerable to attack from the nearby hill. A guards’ union complained this summer that people used the vantage point to toss food, drugs and mobile telephones to prisoners.

The shooting of Lamiri, said to be a senior figure in the Gypsies gang, came after some of the worst multiple shootings seen in France.

The vendetta turned truly bloody last year after a court acquitted five men from the Maghrébin gang who were charged with murdering Lamiri’s brother. In February four men in a fake police vehicle stopped a Maghrébin car and killed three of the occupants, all aged 22. One of the victims had been acquitted of the Lamiri murder.

Prosecutors say that the new-style gangs, based in the ethnic housing estates, which are often no-go areas for police, are difficult to track.

“These kids accept early death as the price for living well,” a police investigator said. “They say they do not expect to live beyond 30. The money goes into beautiful cars, parties with cocaine and prostitutes, sometimes trips to Saint-Tropez to splash out on the beaches. They have been fed on violent TV series and films. They live in the present and they have made death mundane.”

Times Recommends

• Michael Jackson: Not the final chapter
• Indian Army spends10m rupees on golf buggies
• Sacha Baron Cohen forced to tone down Brüno