The man who killed 84 people when he drove a lorry through crowds in the French city of Nice planned the attack for months, and had the help of at least five accomplices, authorities have said.
Paris prosecutor Francois Molins said on Thursday that five suspects - four men and one woman - currently in custody were facing preliminary "terrorism" charges for their alleged roles in helping 31-year-old Mohamed Lahouaiej Bouhlel in the July 14 attack.
Molins' office opened a judicial inquiry into a battery of charges for the suspects, including complicity to murder and possessing weapons tied to a "terrorist" enterprise.
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Bouhlel, a Tunisian national living in Nice, was shot dead by police after ramming a lorry through a large crowd celebrating Bastille Day, France's national holiday.
People close to him previously said that he had shown no signs of holding hardline views until very recently.
But Molins said information from Bouhlel's phone showed searches and photos that suggested he could have been preparing an attack as far back as 2015.
Among the images he viewed were photographs of the July 14, 2015 Nice fireworks display and of a concert on the Promenade des Anglais "focusing on the crowd", he said.
Al Jazeera's David Chater, reporting from Paris, said that prosecutors also revealed that Bouhlel used a "form of amphetamine" also being used by fighters of the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).
Our correspondent said that the latest revelations by prosecutors raised questions about why the French intelligence failed to pick up information from Bouhlel, or his suspected accomplices.
"They had a long time to plan and the intelligence services simply didn't have them on their radar, and that is a very worrying gap in the intelligence community here in France," he said.
READ MORE: Attack in Nice claims victims from around the world
Molins also said that one of the five suspects had filmed the scene of the crime the day after the carnage.
Prosecutors requested the five suspects be charged with conspiracy to commit "terrorism", among other crimes, according to Mollins.
They will be presented to anti-terrorism judges later on Thursday.
The Islamic State and Levant (ISIL also known as ISIS) group claimed responsibility for the attack, though authorities said they had not found any signs that the group had directed it.
France declared three days of national mourning for the Bastille Day massacre, in which the lorry careered for hundreds of metres along the Promenade des Anglais seafront, slamming into families and friends after a firework display marking the anniversary of the 1789 revolutionary storming of the Bastille.
The attack also injured more than 200 people and left the area strewn with bodies, including many children.
Source: Al Jazeera and agencies