A French prosecutor confirmed Tuesday that the man who beheaded his boss and tried to blow up a gas factory had a "terrorist motive" and links to the Islamic State group in Syria.
Yassin Salhi, 35, confessed to the crime but has maintained it was purely for personal reasons and had nothing to do with his religious beliefs, even though it bore all the hallmarks of an extremist act.
"The one does not exclude the other and the choice to kill someone against whom he held a grudge does not exclude a terrorist motive," said Paris chief prosecutor Francois Molins.
Molins pointed to several facts uncovered during the investigation, including links between Salhi and a known French extremist in Syria, who asked permission from the Islamic State group to publish gruesome photos from the attack.
The 35-year-old Salhi, long known to security services for his radical views, was arrested Friday after an attack in which he rammed a delivery van laden with gas bottles into a warehouse containing dangerous chemicals, causing an explosion.
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Firefighters alerted by the blast found him trying to open gas bottles inside the Air Products factory, shouting "Allahu Akbar, before making the grisly discovery of the severed head of Salhi's 54-year-old boss Herve Cornara.
"Salhi decapitated his victim, he hung the head on a fence to get maximum publicity, as he told us during interrogation," said Molins.
"Yassin then tried to blow up gas bottles in what appears to be a martyr operation that could have left a large number of victims," he said, adding there were 75 people on site at the factory.
The first beheading in France during an attack shocked the country just six months after a three-day extremist killing spree in Paris left 17 people dead, most of them gunned down in the editorial offices of satirical magazine Charlie Hebdo.
The prosecutor said Salhi had convinced Cornara, the owner of a delivery company for which he worked, to get into the van with him and later strangled him "with one hand".
He stopped some 500 metres before the factory in the town of Saint-Quentin-Fallavier, some 40 kilometres (25 miles) from the eastern city of Lyon and decapitated Cornara in the back of the van using a knife with a 25 cm blade.
Salhi had also bought a fake gun which he had taken pains to paint on the eve of the attack, showing careful preparation, said Molins.
The head was attached to the fence with a chain and surrounded by two flags belonging to the terrorist organisation which Salhi said he had bought the day before the attack.
"This corresponds very precisely to the orders of Daesh (the Islamic State group) which calls regularly for acts of terrorism on French soil and in particular to cut the throats of unbelievers.
"The decapitation recalls the habitual modus operandi of this terrorist organisation," said Molins.
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Salhi, who refused to talk for his first 24 hours in custody, later gave scattered testimony, claiming he was suffering from amnesia and had no memory of the macabre staging of the severed head.
He also claimed he didn't recall sending two photos of the crime to a French extremist in Syria, including a selfie with the head, and a picture of the body draped in the militant flags with the head posed on the torso.
But investigations showed Salhi was in "regular contact" with the extremist Sebastian Yunis, 30, known to have left for Syria last November.
During a raid on those close to Yunis, who is from Besancon in eastern France, police uncovered a telephone used to contact him in which he said in a Whatsapp conversation on the day of the attack that he indeed knew Yassin and was "one of the reasons he did that", referring to the crime.
Yunis said in the message he had asked the Islamic State group to disseminate the photos taken by Yassin.
Molins said Salhi had spent a year in Syria in 2009 with his wife and children, claiming he was there to learn Arabic.
The married father-of-three first landed on the radar of French security services in 2003 for his connection with extremists and made several visits to Morocco and Saudi Arabia that year and 2004, said Molins.
Authorities said last week that Salhi had been placed on a national security danger watchlist between 2006 to 2008 before being taken off, but continued to raise red flags over his ties to radical movements.