LYON, France (Reuters) - French authorities are questioning a 35-year-old delivery man of North African origin suspected of a macabre Islamist attack involving the beheading of his boss and an attempt to blow up a U.S.-owned chemicals plant in southeastern France.

President Francois Hollande, dealing with new security fears less than six months after 17 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 victim, 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, his wife, sister and a fourth person were being held for questioning over the weekend. Salhi was known to French authorities as a potential risk because he visited Islamists but there has been no claim of responsibility for the attack.

While an anti-terrorist inquiry has been launched, Paris public prosecutor Francois Molins cautioned against premature conclusions and said investigators had yet to fully understand what happened at the industrial zone in Saint Quentin-Fallavier, 30 km (20 miles) 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 at least 37 people at a Tunisian beachside hotel and an Islamic State suicide bomber killed two dozen and wounded more than 200 at a mosque in Kuwait.

However French authorities said there was no connection between the attacks, and there was no indication that the site had been attacked because it belonged to a U.S. company, industrial gases and chemicals group Air Products.

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

"There should be no doubt as to our country's ability to protect itself and remain vigilant," he said, announcing a tightening of national security to levels he said were unprecedented in recent decades.

Unlike two of gunman behind the January attacks, Salhi does not have a criminal record, but the fact he was listed between 2006 and 2008 as someone at risk of radicalization, and later came to the attention of intelligence services because of his links to radical Islamists, will spark local political debate.

It could also rekindle tensions surrounding France's five million Muslims, despite the fact that the vast majority of them were repulsed by the Charlie Hebdo killings.

While Salhi's Islamist connections were known to authorities, neighbors at his family home in a quiet Lyon suburb expressed disbelief at the turn of events on Friday.

"They are a very normal family," a 46-year-old housewife who gave her name as Brigitte said. "I only talked with madame, he didn't say hello or goodbye."

(Writing by Mark John; editing by Clive McKeef)