Updated: 19.15
Mohamed Bouhlel killed 84 people after driving a 19-ton truck into a crowd which had been watching a fireworks display on the seafront during Bastille Day celebrations.
According to prosecutor Francois Molins, a search of Bouhlel's phone revealed searches and photos that indicated he had been plotting an attack since 2015.
"The investigation under way since the night of July 14 has progressed and not only confirmed the murderous premeditated nature of Mohamed Lahouaiej Bouhlel's act but also established that he benefited from support and complicity," Mr Molins said.
Five suspects remain in custody and are facing preliminary terrorism charges for their alleged roles in helping French-Tunisian Bouhlel.
Mr Molins told a news conference in Paris that one of the suspects filmed the scene of the crime the day after the carnage.
The five suspects, who had not been known to intelligence services, have been named as: 21-year-old French Tunisian Ramzi A, 37-year-old Tunisian Chokri C, 38-year-old Albanian Artan H, 40-year-old French Tunisian Mohamed Oualid G, and 42-year-old French Albanian Enkeledja Z.
Bouhlel received a text from one of the alleged accomplices a few days after the January 2015 Islamist attacks on the Charlie Hebdo newspaper and a Jewish deli in Paris saying: "I am not Charlie ... I am glad they brought in Allah's soldiers to finish the job".
In the wake of the attack, people around the world adopted the slogan "Je Suis Charlie" (I am Charlie) in solidarity with the victims at the satirical newspaper.
More than 400 investigators have been poring over evidence since the deadly attack in the southern French coastal city of Nice, which also left 300 injured.
France's interior minister Bernard Cazeneuve has acknowledged there was no national police presence at the entrance to the promenade at the time.
He said only local police, who are more lightly armed, were guarding the walkway.
Meanwhile, counter-terrorism police have carried out raids in a region just north of Paris on Thursday.
French justice officials said the swoop was not directly connected with the attack in Nice.
