The 23-year-old, identified only as Ilan, was discovered on Monday near a train station in the Paris suburbs.

The shop assistant, who was found naked, bound and gagged, and covered in burns died on the way to hospital. He went missing in January after a date with an unknown woman, who approached him at his workplace in central Paris.

Police said they had recorded at least four other similar kidnap attempts since December.

Twelve of those held overnight, aged 17 to 32, were arrested on an estate in the suburban town of Bagneux, where police say the victim was held and tortured. Another suspect was arrested in Belgium.

Lure

Ilan's kidnappers sent a 400,000 euro (£273,500; $475,000) ransom demand in e-mails and text messages to his family, which they were unable to raise.

The sum later dropped to 5,000 euros, after which the gang broke off contact.

The man was found on Monday morning near the Sainte-Genevieve-des-Bois train station.

On Thursday, a young blonde woman told police that she had recognised herself in a computer-generated portrait of a suspect circulated to the press.

She confirmed that she had been asked to entice several young men, but had not known why. She has also been detained.

Dangerous ringleader

Paris public prosecutor Jean-Claude Marin told a press conference that the presumed leader of the organised gang was still on the run, but that he had been identified and was "extremely dangerous".

Aged 26, he calls himself the "brain of the barbarians" and is thought to have had a number of "run-ins" with the law.

The kidnapping has alarmed France's Jewish community, since the victim worked in a Jewish neighbourhood in Paris, and he and several of the others targeted were Jewish.