Mohamed Lahouaiej Bouhlel, who mowed down crowds of people enjoying a Bastille Day fireworks display, had long plotted the carnage, prosecutor Francois Molins said.
The revelations come as the French government continues to be plagued by questions over possible security failings, prompting authorities to launch an investigation into potential oversights.
Molins said photos on Bouhlel's phone showed he had likely already staked out the same July 14 event in 2015.
It also emerged that one of the five suspects in custody, a Tunisian named Mohamed Oualid G., had filmed the scene the day after the carnage, as it crawled with paramedics and journalists.
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They are 22-year-old Franco-Tunisian Ramzi A., 37-year-old Tunisian Chokri C., 40-year-old Tunisian Mohamed Oualid G., 38-year-old Albanian Artan H., and his wife Enkeledja Z. Who holds both French and Albanian nationality.
None were known to intelligence services, and only Ramzi A., who was born in Nice, had a criminal record, for robbery and drug offences.
He led police to discover a Kalashnikov and a bag of ammunition yesterday, however the purpose of the weapons was unclear.
Ramzi, Chokri and Oualid were charged with being accomplices to murder by a terror group.
More than 400 investigators have been poring over evidence since the grisly attack last Thursday, the third in France in 18 months, and it was analyses of Bouhlel's telephone records that led them to the five suspects.
While the Islamic State group claimed the attack, describing Bouhlel as a "soldier", investigators have not found direct proof of his allegiance to the jihadists.
However initial details of the investigation revealed Bouhlel had been fascinated with jihad for a while.
In May last year, he took a photo of an article about the drug Captagon, an amphetamine used by jihadists in Syria.
In July 2015 he took photos of the crowd at the Bastille Day fireworks display, as well as another crowd watching a concert on Nice's Promenade Des Anglais three days later.