By Michel Rose
LYON, France, June 27 (Reuters) - 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 U.S.-owned chemicals plant in southeastern France.
President Francois 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 on Saturday, according to a law-enforcement source. His wife, sister and a fourth person are also in detention.
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 Francois 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 (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.
THREE ATTACKS ON SAME DAY
The latest attack in France occurred on the same day that a gunman killed 39 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 established connection between the attacks and no indication that the site had been targeted because of its U.S. owner, 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 the gunmen behind the January attacks Salhi does not have a criminal record. But the fact he was formally flagged as a risk between 2006 and 2008 and known to have since maintained radical Islamist links has raised new questions about the security services and their effectiveness.
Opposition leader and former president Nicolas Sarkozy, who was in office when Salhi's surveillance was lifted in 2008, said on Friday his conservative Les Republicains party had been urging the government "for several weeks" to tighten security.
The attack may also renew tensions surrounding France's five million Muslims, despite the widespread revulsion expressed by many Islamic leaders and communities over 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," she added. (Additional reporting by Nicolas Bertin; Writing by Mark John; Editing by Clive McKeef and Greg Mahlich)