The investigation showed Mohamed Lahouaiej Bouhlel had trawled the internet for information on a terror attack in the US city of Orlando and on the killing of a police couple in a Paris suburb last month, Francois Molins said.
A search of his computer also found violent images "linked to radical Islam", he told a press conference in Paris.
However he said no clear link had been established between the father of three and the Islamic State group which claimed the Bastille Day carnage.
The revelations came on the third day of mourning over the grisly attack, which saw Bouhlel use a 19-tonne truck to mow down crowds leaving a Bastille Day fireworks display in the French Riviera city.
A sea of people thronged the seafront promenade in Nice where the attack took place for an emotional minute's silence. Similar ceremonies were held across the country, accompanied by the ringing of church bells.
But in a sign of the anger and bitterness gripping France after its third major terror attack in 18 months, Prime Minister Manual Valls was booed as he arrived and left Nice for the tribute.
Valls dismissed the jeers and calls for him to resign as "disgraceful", saying they reflected the "attitude of a minority" in the city run by the opposition Republicans party.
Molins said the investigation confirmed the attack was "premeditated", and said 13 victims had yet to be identified.
Photographs found on the attacker's mobile phone showed he staked out the promenade in the days before he struck, he added.
Molins painted a picture of a non-practising Muslim who ate pork, drank, took drugs and had an "unbridled sexual activity".
Interior Minister Bernard Cazeneuve defended the government efforts to halt terror attacks, calling for "dignity and truth" from critical opposition politicians as the national mood sours further nine months ahead of the next presidential election.
The Nice attack came eight months after IS jihadists killed 130 people across Paris, and 18 months after three days of terror at the Charlie Hebdo weekly and a Jewish supermarket killed 17.
Former president and main opposition leader Nicolas Sarkozy said on Sunday that "everything that should have been done the past 18 months was not done".
"We are at war, outright war. So I will use strong words: it will be us or them," he said.
While previous attacks saw grand displays of national unity, there was no semblance of cohesion after the Nice massacre, with Sarkozy joining a long line of opposition politicians who have accused the government of not doing enough to protect the French.
Cazeneuve described the bitter debate as "shameful".
"Certain members of the political class have not respected the mourning period," he said.
The frustration of the French was writ large in some of the messages left among flowers and tributes on Nice's seafront.
"Enough with the speeches" and "Sick of carnage in our streets", the messages read.
Cazeneuve sought to highlight the measures taken by government to fight terrorism, such as boosting the armed forces and adopting new anti-terror laws.
"There is no zero risk. By saying this we are telling the truth to the French and tackling the threat with lucidity," he said.
After the latest attack, the government called for volunteers to become reservists who can be called on to supplement the security forces -- already on high alert under an eight-month-old state of emergency.
Text messages and selfies
The deadly use of an easily obtainable vehicle as a weapon by a man who had no long history of radicalisation has highlighted the challenge for intelligence and security officials in stopping such attacks.
"We cannot exclude that an unbalanced and very violent individual" had been "through a rapid radicalisation," Cazeneuve said.
As investigators piece together details about his motives and planning, it emerged that Bouhlel had used the rented truck to stake out the Nice seafront for two consecutive days before striking.
A source close to the investigation told AFP that he had also sent a text message just before the attack in which he "expresses satisfaction at having obtained a 7.65-millimetre pistol and discusses the supply of other weapons".
He also took a selfie at the wheel of the truck in the days before the attack.
Six people were in custody on Monday including a 38-year-old Albanian suspected of providing Bouhlel with a pistol he used to fire at police during the attack.
In Nice, many people were still desperately waiting for news of their loved ones.
Molins said 71 victims had been officially identified so far, with the authorities taking painstaking measures to avoid errors of identification seen during the Paris attacks last November.
At least 10 children were among the dead, as well as tourists from Ukraine, Switzerland, Germany, and a local Russian association said there were about 10 victims from Russia.
Eighty-five people were still being treated in hospital on Sunday, 18 of them in critical condition.