The lorry driver who killed 84 people on a Nice beachfront had accomplices and had been plotting his attack for months, according to the prosecutor leading the anti-terror investigation.
Prosecutor Francois Molins said five suspects were facing preliminary terrorism charges for their alleged roles in helping driver Mohamed Lahouaiej Bouhlel.
In a judicial inquiry which opened on Thursday, Mr Molins said information from Mr Bouhlel's phone showed searches and photos that indicated he had been preparing an attack since 2015.
Isis has claimed responsibility for the attack, in which Mr Bouhlel drove a lorry into crowds of people gathered on the Promenade Des Anglais in the French city for Bastille Day celebrations on 14 July.
More than 200 people were injured after Mr Bouhlel drove more than one mile in the 19-tonne hired truck, sending hundreds fleeing for their lives.
Mr Bouhlel, 31, was shot dead by police at the scene.
Four men and one woman are currently in custody, according to France Info. "They are not known by the intelligence services," Mr Molins confirmed.
The youngest suspect is 22 years old and was born in Nice, with dual French and Tunisian nationality.
He was known by police and had six minor convictions between April 2013 and May 2015.
The other four detainees are aged between 37 and 42. One man and one woman are Albanian, while the other two are of Tunisian descent.
None of the other four suspects have criminal records.
"The investigation will endeavour to determine the links, which have not yet been established, between the perpetrators of the attack, and especially with those acting in the name of the terrorist organisation Daesh [Isis], who has claimed responsibility for this act," said Mr Molins.
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