Anti-terror police will grill the suspect, Yassin Salhi, a 35-year-old father-of-three, as they search for clues and a motive for Friday's attack on a gas warehouse near France's second city of Lyon.
After several hours of silence, Salhi has begun to open up to investigators about the assault, which came six months after 17 were killed in Islamist attacks in Paris that began with the Charlie Hebdo massacre.
And Canadian authorities are trying to help France solve the case after it emerged Salhi sent a gruesome selfie photo of himself and the severed head to a WhatsApp number in Canada.
The probe is naturally focusing on Syria, where hundreds of French people have gone to wage jihad, officials said.
Anti-terrorist authorities have identified 473 people who have left France to fight in Iraq or Syria, according to sources close to the probe.
On Friday morning, Salhi rammed his van into the US-owned Air Products factory in what President Francois Hollande said was a "terrorist" attack designed to blow up the whole building.
He was overpowered by a firefighter as he was trying to prise open a bottle of acetone in an apparent suicidal bid to destroy the factory.
The attack came on a day of bloodshed on three continents that saw 38 people mown down on a Tunisian beach and 26 killed in a suicide attack in Kuwait.
The Islamic State extremist group has claimed responsibility for those two attacks but no group has said it carried out the French operation.
Sources close to the investigation said Salhi was radicalised more than a decade ago after contact with Muslim convert Frederic Jean Salvi - known as "Ali" - who is suspected of preparing attacks in Indonesia with Al-Qaeda militants.
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