Mohamed Lahouaiej-Bouhlel was a petty criminal prone to depression and violence who smoked, drank and never went to the mosque, according to neighbours and family.
The Tunisian delivery driver, who on Thursday evening smashed a 19-tonne lorry into a crowd killing 84 people, including 10 children, had shown no overt signs of radicalistaion.
The Islamic State group, in claiming the attack today, said he was a "soldier" who had responded to "calls to target nations of coalition states that are fighting (IS)".
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But authorities said he had not been flagged for links to radical Islamic ideology.
The 31-year-old "seemed to have been radicalised very quickly from what his friends and family" have told police, French Interior Minister Bernard Cazeneuve said today.
Speaking outside his home in Msaken, eastern Tunisia, the attacker's father said he 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 he shouted... He would break anything he saw in front of him," Mohamed Mondher Lahouaiej-Bouhlel said.
"We are also shocked," he said, adding that he had not seen his son since he left for France but was not entirely sure when this was.
"He didn't pray, he didn't fast, he drank alcohol," his father said. "He even took drugs."
After Thursday's attack on people who had just enjoyed a Bastille Day fireworks display on Nice seafront, his neighbours in a working-class neighbourhood of the city told AFP they had little to do with him.
They portrayed him as a solitary figure who rarely spoke and did not return greetings when their paths crossed.
One neighbour in his four-storey block said she had concerns about him, describing him as "a good-looking man who kept giving my two daughters the eye".
However, another said she had become friendly with him after they struck up a conversation one day in the stairwell when he was looking to buy some cigarettes.
According to her he was teetotal and refused to drink a glass of wine with her, saying: "No, I don't drink."
The woman, who spoke on condition of anonymity, also said he asked to rent her letter box from her.
"I don't know why. I found it strange, quite frankly," she said, adding that she turned down his request.
Lahouaiej-Bouhlel was often seen drinking beer and never attended the small mosque near his home, other residents of his home district told AFP.
