French authorities are questioning a 35-year-old delivery man of North African origin over a suspected Islamist attack involving the beheading of his boss and an attempt to blow up a US-owned chemicals plant in southeastern France.

President François Hollande, dealing with new security fears less than six months after 17 people were killed by Islamist gunmen at satirical weekly Charlie Hebdo and a Jewish foodstore in Paris, said the incident clearly amounted to a terrorist attack.

Yassim Salhi is suspected of having rammed his delivery van into a warehouse of gas containers, triggering an initial explosion. He was arrested minutes later while opening canisters containing flammable chemicals, prosecutors said on Friday

Police later found the head of the 54-year-old manager of the transport firm that employed the suspect, dangling from a fence at the site, framed by flags with written references to Islam. Salhi is being held in Lyon, where he was refusing to respond to interrogators yesterday, according to a law-enforcement source. His wife, sister and a fourth person are also in detention.

There is no other link other than to say that terrorism is our common enemy

Salhi is known to have associated with Islamists over more than a decade and had previously been flagged by French authorities as a potential risk, but there has been no claim of responsibility for the attack.

While an anti-terrorist inquiry has been launched, Paris public prosecutor François Molins said it would be premature to make any conclusions at this stage and investigators had yet to fully understand what happened at the industrial zone in Saint Quentin-Fallavier, 30 km south of the city of Lyon.

“Questions remain over the exact chronology of events, what happened when he arrived, the circumstances of the decapitation, the motivation and whether there were accomplices,” he said.

The latest attack in France occurred on the same day that a gunman killed 39 people on a Tunisian beach and an IS suicide bomber killed two dozen and wounded more than 200 at a Kuwait mosque.

“There is no other link other than to say that terrorism is our common enemy,” said Hollande after rushing back to Paris from an EU summit in Brussels.