PARIS — Mohamed Lahouaiej Bouhlel, who killed 84 people in a terrorist attack in Nice, France, last week, planned his assault over several months and got help from at least five people, the Paris prosecutor said on Thursday.

However, although the Islamic State called the attacker one of its “soldiers,” there is as yet no evidence that he or the suspected accomplices had any direct contact with the terrorist network, the prosecutor, François Molins, who handles terrorism investigations in France, said at a news conference in Paris.

Mr. Lahouaiej Bouhlel, a 31-year-old Tunisian who lived in Nice, drove a cargo truck through crowds that had gathered on the city’s waterfront promenade to watch the Bastille Day fireworks on July 14. He also fired an automatic pistol at the police, before they shot and killed him.