AFP reporters interviewed about a dozen neighbours of the man, named by police as 31-year-old Franco-Tunisian Mohamed Lahouaiej-Bouhlel, whose identity papers were found in the truck.
They portrayed him as a solitary figure who rarely spoke and did not even return greetings when their paths crossed in the four-storey block, located in a working-class neighbourhood of Nice.
Sebastien, a neighbour who spoke on condition that his full name was not used, said Lahouaiej-Bouhlel did not seem overtly religious, often dressed in shorts and sometimes wore work boots.
He had a van parked nearby and owned a bike, which he brought up into his first-floor apartment.
Of those who were interviewed, only one, a neighbour on the ground floor, said she had had any concerns about him — he was “a good-looking man who kept giving my two daughters the eye.”
Police investigators and forensic experts entered his apartment around 9.30am (0730 GMT) with an armed police intervention unit in support, and brought out bags of material later.
At least 84 people were killed when the white 19-tonne truck slammed into the crowd on the Promenade des Anglais, Nice’s glitzy beachfront, as they gathered to watch a firework display on France’s national day.
A source close to the investigation said an “inactive” grenade was found inside the vehicle, as well as “several fake rifles.”
President Francois Hollande has declared the attack was of “an undeniable terrorist nature.”
“Investigations are currently underway to establish if the individual acted alone or if he had accomplices who might have fled,” interior ministry spokesman Pierre-Henry Brandet said.