French authorities were transferring the man suspected of beheading his boss in an alleged jihadist attack to Paris on Sunday, as it emerged he sent a macabre selfie photo of the decapitation.

Yassin Salhi, a 35-year-old father-of-three, has admitted killing his boss, a source close to the investigation said on Sunday.

Salhi told detectives he had killed Herve Cornara in a parking area before arriving at the plant in Saint Quentin-Fallavier, 30 km south of Lyon, where he attempted to cause an explosion on Friday.

Anti-terror police searched for clues and a motive for Friday's attack.

Salhi was apprehended minutes later while opening canisters containing flammable chemicals.

After several hours of silence, Salhi has begun to open up to investigators about the assault that came six months after 17 were killed in Islamist attacks in Paris that began with the Charlie Hebdo massacre.

Canadian authorities are trying to help France solve the case after it emerged Salhi sent a gruesome photo of himself and the severed head to a WhatsApp messaging app number in Canada.

Investigators have warned, however, that it could be a relay number, and the intended recipient could be anywhere in the world.

The probe is 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.

AFP - Reuters