The suspect accused of decapitating his boss and attempting to blow up a gas plant in southeastern France had taken a macabre selfie with the severed head before his arrest, a source close to the investigation said on Saturday.

Yassim Salhi, 35, is being held in the city of Lyon after he rammed his delivery van into a warehouse storing gas containers and hung his employer's severed head on a factory gate on Friday.

Examination of Salhi's cellphone revealed that he had sent his self-portrait via the Whatsapp messaging application to a user account associated with a North American number, an anonymous law enforcement source told Reuters on Saturday. The ownership of the account was not yet clear.

The Associated Press reported Saturday that unnamed officials, who requested anonymity because the investigation is ongoing, said the image was sent to a Canadian number. 

Jean-Christophe de Le Rue, a spokesman for Public Safety Minister Steven Blaney, did not confirm to CBC News that the image was sent to a Canadian number, but did say Canadian authorities are involved in the case. 

"While I cannot comment on operational matters of national security, we are assisting the French authorities with their investigation," he said in an email statement. 

He declined to comment further on details of the investigation.

Police found the head of Herve Cornara, manager of the transport firm that employed Salhi, attached to a fence at the plant owned by U.S. group Air Products, framed by black and white flags bearing Islamic slogans.

No claim of responsibility for attack

President Francois Hollande, dealing with new security fears six months after Islamist gunmen killed 17 people at the offices of satirical weekly Charlie Hebdo and a Jewish deli, said the incident clearly amounted to a terrorist attack.

Salhi, whose wife and sister are also being detained along with a fourth person, is known to have associated with Islamists over more than a decade and had previously been flagged as a potential risk.

But there has so far been no claim of responsibility for the attack.

France attack site

The attack occurred in Saint-Quentin-Fallavier, in the Isère region between Lyon and Grenoble. (CBC)

Paris public prosecutor Francois Molins said on Friday that investigators would take time to piece together the sequence of events at the plant in Saint Quentin-Fallavier, 30 kilometres south of Lyon, and establish whether Salhi was acting alone.

The body of Cornara, 54, also bore signs of strangulation suggesting that he may have been killed at an unknown location prior to his decapitation.

Investigators searching his home in Saint-Priest, near Lyon, seized a laptop, a tablet computer and a fake pistol but found no trace of explosives or radical propaganda.

"We don't know whether we're dealing with a fundamentalist who just lost it or a real terrorist," the source said.

3 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 ISIS suicide bomber killed two dozen and wounded more than 200 at a mosque in Kuwait.

French authorities said there was no connection between the attacks and nothing to indicate that the industrial site had been targeted because of its U.S. ownership.

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

Unlike two of the gunmen behind the January attacks, Salhi does not have a criminal record. But the fact he was flagged as a risk between 2006 and 2008 and known to have since maintained radical Islamist links is raising new questions about the security services and their effectiveness.

With files from The Associated Press and CBC News