An Islamist terrorist tied his boss's severed head to a fence before making a failed ram raid on a chemical factory near Lyon yesterday.
Investigators believe the attack was intended to take scores of lives.
Images caught on security cameras of the attacker placing the head on the factory fence before the attack have shocked France. The head was wrapped in an Isil banner carrying a message in Arabic.
The victim was the 45-year-old manager of a local transport company, which had permission to enter the heavily protected site. The attacker was one of his drivers, Yassin Salhi (35), a French-born father of three.
The rest of the manager's body was found after the attack outside the Air Products factory at Saint-Quentin-Fallavier, in the south-east suburbs of Lyon.
Security camera tapes showed his employee - who had previously been identified by French authorities as an Islamist - attaching his boss's head to a chain-link fence in what investigators described as a "macabre piece of theatre".
Salhi then climbed back into the company truck and entered the factory compound in the normal way. Once inside the site, he drove at high speed into a pile of gas-canisters, apparently intending to cause a huge explosion.
Luckily, the resulting blast was relatively small. Two factory employees were injured. Salhi survived with a minor head injury.
After the blast he ran into one of the factory buildings to try to tamper with other gas canisters. When fire fighters arrived he shouted "Allahou Akbar" (God is the greatest) before he was overpowered.
Although less disastrous than it might have been, the attack sent shock waves through France less than six months after the jihadist attack on Charlie Hebdo in Paris in January.
President François Hollande left the EU summit in Brussels to chair a crisis meeting of his defence council in Paris. He appealed for calm in the face of the continuing jihadist threat. It was vital, he said, that the country did not give way to "fear" and "useless divisions and intolerable suspicions".
Police also arrested a suspected accomplice, who was seen driving up and down the road near the factory yesterday morning. They also took into custody Salhi's wife and another woman at his flat.
Before she was taken into custody - possibly for her own protection - Salhi's wife expressed her disbelief at his alleged actions. "What's happening here? My heart is going to break. It can't be him," she said. "We are a normal family with children. We live normally."
It is understood that Salhi had previously been under surveillance by the French security services for alleged involvement with Salafist Islamic radicals, but was taken off the watch list after two years.
Investigators believe that the Air Products factory was targeted because it was US-owned and was assumed to contain highly explosive chemicals. (© Daily Telegraph, London)