Tunisian attacker Mohamed Lahouaiej-Bouhlel visited the Nice promenade with his rented truck on the two days before he rammed the vehicle into a crowd, killing 84, a source close to the investigation said Sunday.
The source said police had questioned hundreds of people since Thursday night’s attack, several of whom said Lahouaiej-Bouhlel did show signs of being religious.
French authorities have said the attacker became radicalised “very quickly”, and some of his family and friends had previously said he smoked, drank and never went to the mosque.
Lahouaiej-Bouhlel’s neighbours and father earlier described him as a violent depressive who showed no outward signs of being a devout Muslim.
The Islamic State group claimed responsibility for the killings on Saturday, three days after the assault.
In a statement via its Amaq news service, IS said one of its “soldiers” carried out the attack “in response to calls to target nations of coalition states that are fighting (IS)”.
After crisis talks in Paris, French Defence Minister Jean-Yves Le Drian noted that IS had recently repeated calls for supporters to “directly attack the French, Americans, wherever they are and by whatever means”.
“Even when Daesh is not the organiser, Daesh breathes life into the terrorist spirit that we are fighting,” he said, using an Arabic name for IS.
Police said they had arrested four more people linked to Lahouaiej-Bouhlel, in addition to his estranged wife who was taken into custody on Friday.
Cazeneuve said the father-of-three “seemed to have been radicalised very quickly, from what his friends and family” have told police.
“We are now confronted with individuals open to IS’s message to engage in extremely violent actions without necessarily having been trained or having the weapons to carry out a mass (casualty) attack,” he said.
IS also claimed responsibility for the November 13 attacks which killed 130 people in Paris, while gunmen in January 2015 attacks on the Charlie Hebdo weekly and a Jewish supermarket were linked to both IS and al Qaeda.
A French parliamentary inquiry last week criticised numerous failings by the intelligence services over the Paris massacres.
France is also home to hundreds of militants who have flocked to fight alongside IS.