Video report by ITV News Correspondent Paul Davies
Mohamed Lahouaiej Bouhlel has been named by local media as the terrorist who used a 19-tonne lorry to mow down a crowd in Nice.
Here's what we know about his background and possible motive for murdering at least 84 people.
Who is he?
Mohamed Lahouaiej Bouhlel is a 31-year-old Tunisian who had lived in France for 10 years.
He did not have French nationality but held a French residence permit and worked as local delivery driver.
Bouhlel was married with three children but had marital problems, according to Tunisian security sources.
He was from the town of Msaken - six miles outside Sousse where a gunman killed 38 people in 2015 - but the last time he is believed to have gone home was four years ago.
What did he do?
Bouhlel ploughed into crowds at speeds of over 50mph killing 84 people - including 10 children and injuring 202 others, 52 of whom are in a critical condition.
Witnesses said he swerved from side to side "hitting people like skittles" to kill as many as possible.
Records show he rented the 19-tonne he used to carry out the atrocity just two days before the massacre.
Police said he fired three shots at them before they returned fire, eventually killing him and bringing the vehicle to a stop.
Bouhlel's identity was later confirmed by his fingerprints.
Was he previously known to the authorities?
Bouhlel was known to police as a petty criminal with a history of domestic violence.
But he only had one conviction for road rage dating back to March this year, French Justice Minister Jean-Jacques Urvoas said.
Urvoas added: "There was an altercation between him and another driver and he hurled a wooden pallet at the man."
As it was his first offence, Bouhlel was given a suspended sentence and had to contact police once a week, which he did.
Is he linked to any terror groups?
Bouhlel was not being watched by French intelligence services and and is not known to have any extremist views or links to Islamist groups.
So-called Islamic State have since claimed responsibility for the attack.
But ITV News Security Editor Rohit Kachroo said it should be treated with caution as Bouhlel does not fit the usual terrorist profile.
He said there was no evidence to suggest Bouhlel had been radicalised,
Bouhlel's ex-wife is currently being questioned by police.
What evidence have police found?
Police found various weapons, including fake guns, in the lorry Bouhlel used to carry out the attack.
They included two automatic pistols and two replica Kalashnikovs which were recovered from the vehicle's cabin.
A bicycle was also found in the back of the lorry, police said.
Officers are now searching his house where they have recovered a phone and a computer.
What do people who knew him say?
Bouhlel's brother told ITV News the family had no idea of what he was planning or what had prompted him to kill so many people.
And friends described him as a "recluse" who may have been mentally disturbed.
Neighbours who lived in the same block of flats as Bouhlel said he drank and didn't seem particularly religious.
One woman told ITV News: "He looked at us weirdly and wouldn't open his door. I just thought 'not normal'."
According to the French prime minister, Manuel Valls, Bouhlel was radicalised "very quickly".
While no solid proof of links between Bouhlel and Islamic State have been found during initial investigations, Mr Valls said there was no doubt about the assailant's motives - extremism.
You can follow the latest updates on the Bastille Day attack here.