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PARIS, July 21 (Xinhua) -- Mohamed Lahouaiej Bouhlel, a delivery man who killed 84 people with a heavy truck in Nice last week, prepared for months with the help of accomplices, Paris prosecutor Francosi Molins said Thursday.

Photo taken on July 15, 2016 shows a black ribbon on a French national flag in Nice, France. People in the wake of the deadly Nice attack on Thursday, French Prime Minister Manuel Valls on Friday morning declared three days of national morning, to begin on July 16. (Xinhua/Xu Jinquan)

"The investigation underway since the night of July 14 has progressed and not only confirmed the murderous premeditated nature of Bouhlel's act but also established that he benefited from support and accomplices," Molins said.

The 31-year-old man seemed to "have planned his criminal project several months before passing to act," he added.

Molins said four men and a women not known to intelligence services were "involved in the preparation" of the attack. They have been placed under formal investigation.

Bouhlel had exchanged thousands of calls and text messages last year with the accomplices: a Tunisian resident, two Franco-Tunisian men and an Albanian couple.

The attacker had "certain recent interest in radical jihadist movements," but no evidence has been found on links with the Islamic State (IS).

The Islamic State has claimed responsibility for the assault, saying Bouhlel one of its soldiers.

Eight months after gunmen killed 130 people in Paris, the Nice attacker plowed a heavy truck into crowds on Bastille Day, killing 84 people and injured 300 others before being shot dead by police officers.