NICE (France) • The French authorities said the man who ploughed a truck through crowds watching Bastille Day fireworks in the Riviera city had no known connection to militant groups, even as Islamic State in Iraq and Syria (ISIS) yesterday claimed responsibility for the assault.

Mohamed Lahouaiej-Bouhlel, a 31-year-old Tunisian, was known to law enforcement for petty theft and domestic violence.

But he was not on any list of people known to be radicalised.

He "had not been known to the intelligence services because he did not stand out... by being linked with radical Islamic ideology," Interior Minister Bernard Cazeneuve said yesterday.

If he was a Islamist militant, he must have become radicalised very quickly, Mr Cazeneuve added.

Family members revealed that the attacker suffered from psychological problems.

Speaking in Msaken, eastern Tunisia, his father Mohamed Mondher said his son had suffered from depression and had "no links" to religion. "From 2002 to 2004, he had problems that caused a nervous breakdown. He would become angry and... would break anything he saw," he said.

His sister Rabeb said he had been seeing psychologists for several years before he left home for France in 2005.

Lahouaiej-Bouhlel rented a 19-tonne refrigerated truck and drove it along the city's beach promenade, plowing more than 2km into crowds celebrating Bastille Day on Thursday.

At least 84 people were killed and more than 200 injured.

"He carried out the operation in response to calls to target nationals of states that are part of the coalition fighting the Islamic State," said the news agency Amaq, which supports the militant group.

Police had arrested four more people linked to Lahouaiej-Bouhlel, as well as his estranged wife.

A resident of the apartment block where the family had lived until 18 months ago before they split up said Lahouaiej-Bouhlel had an extreme reaction to his wife's request for a divorce after a violent argument.

"He defecated everywhere, he cut up his daughter's teddy bear and slashed the mattress,'' said the man, who asked not to be identified.

"I don't think there was a radicalisation issue. I think there was psychiatric problem," he said.

Top French anti-terror prosecutor Francois Molins said the attacker had a significant record of crime and violence that stretches back six years. In March, he was sentenced to six months in prison for assault with a weapon in an attack in January. It was not clear whether he had served any part of that sentence.

The motive of the truck rampage was also unclear as he did not leave behind a declaration of intent.

The attack was the third major assault in France in 18 months and anger is mounting over the country's security measures.

"We now realise that there was no protection for us," said Mr Karim Lourahri, 22, a butcher who witnessed the truck mow down several people.

Many questioned how the attacker could have swept past police checkpoints at a prominent event that clearly demanded high security.

Witnesses said there were barriers to prevent vehicles from entering the avenue but they were flimsy. Some said their bags were not checked for bombs.

French President Francois Hollande met with his defence and security chiefs, and Cabinet ministers yesterday.

A spokesman quoted him as saying that in the face of "these attempts to divide the country... we should remember the unity and cohesion of the country around our values".

AGENCE FRANCE-PRESSE, REUTERS, WASHINGTON POST, NEW YORK TIMES