The man behind the Bastille Day truck attack in Nice planned the atrocity for months and was helped by accomplices, French prosecutors have said.

Mohamed Lahouaiej Bouhlel killed 84 people when he drove his truck through crowds gathered to watch a fireworks display on the Promenade des Anglais last week.

Bouhlel was shot dead by police at the scene, but five suspects have since been arrested and are facing preliminary terrorism charges for their alleged roles in helping the driver.

Prosecutor Francois Molins said his office, which oversees terrorism investigations, has information from Bouhlel's phone which shows searches and photos indicating he had been studying an attack since 2015.

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