Story highlights Prosecutor: Computer, cell phone analysis shows attacker had plotted since 2015

One suspect allegedly sent driver message: "Load the truck with tons of iron and cut the brakes."

(CNN) The man who drove a truck into a crowd in Nice, France, killing 84 people, plotted his July 14 attack for months with "support and accomplices," Paris prosecutor Francois Molins said Thursday.

Molins said authorities are investigating five suspects who are in custody on terror charges related to the attack that Mohamed Lahouaiej Bouhlel carried out on Bastille Day, France's main national holiday.

One suspect allegedly sent Bouhlel a Facebook message saying, "Load the truck with tons of iron and cut the brakes. I'll look brother."

Bouhlel, 31, drove a 20-ton truck through hundreds who had gathered to watch the Bastille Day fireworks on the Mediterranean city's waterfront.

Police are at the scene with the truck used in the Bastille Day attack on Nice's Promenade des Anglais.

An analysis of cell phone and computer content -- including a photo of last year's Bastille Day fireworks event focused on the crowd -- showed Bouhlel had been planning the assault since at least 2015, Moins said.

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