The first man arrested over today’s attack in France was known to intelligence services for possible ties to Islamic extremists but had no arrest record, interior minister Bernard Cazeneuve has said.
A decapitated head was found posted on the gate at the entrance to a gas factory in an industrial site in Saint-Quentin-Fallavier, south-east of Lyon, after it came under attack, security officials said earlier.
Mr Cazeneuve added that the investigation has just begun and cautioned against jumping to conclusions.
The suspect had been flagged as an extremist in 2006 and then police monitoring dropped off two years later, according to Mr Cazeneuve.
Several people are in custody after the attack, Mr Cazeneuve said.
Authorities are still trying to identify the victim, he added.
“People who could have participated in this abject crime are in custody after having been arrested,” he told journalists.
France’s anti-terror prosecutor said the attack was carried out by “a terrorist group”.
The industrial site belongs to Air Products, a US chemical company based in Pennsylvania.
The torso was found near the site of an explosion at the scene near Grenoble but the victim was not decapitated by the blast, officials said.
Two flags, one white and one black and both with Arabic inscriptions, were found at the scene.
Several people were wounded in the incident. French officials said it began mid-morning in the Isere region when two men crashed a car into the entrance of the factory and into gas canisters, touching off the explosion.
One attacker, who is said to be known to authorities, has been arrested.
President Francois Hollande described it as a terrorist attack and said one suspect had been arrested and a second might have been involved.
He is returning to France and called a high level defence council meeting.
France went on high alert in January after extremist brothers Said and Cherif Kouachi killed 12 people – including two police officers – in attacks on satirical magazine Charlie Hebdo in Paris.