Five suspects have been charged over the Nice beachfront truck attack that killed 84 people.
The news came after the Paris prosecutor said that the truck driver had accomplices and appeared to have been plotting his attack for months.
Francois Molins cited text messages, more than 1,000 phone calls and video of the attack scene on the phone of one of five people facing terror charges.
The five were handed preliminary terrorism charges on Thursday night for their alleged roles in helping 31-year-old Mohamed Lahouaiej Bouhlel (pictured) in the July 14 attack in the southern French city.
The prosecutor's office, which oversees terrorism investigations, opened a judicial inquiry into a battery of charges for the suspects, including complicity to murder and possessing weapons tied to a terrorist enterprise.
Details about the investigation came as France's interior minister faced criticism that a faulty security plan may have opened the way for the truck attack, and as France extended its state of emergency for six months.
The prosecutor said the investigation had made "notable advances" since the Bastille Day attack by Bouhlel, a Tunisian who had been living legally in Nice for years.
Bouhlel was killed by police after driving a truck down Nice's famed Promenade des Anglais, mowing down those who had come to see holiday fireworks.
The detained suspects are four men - identified as Franco-Tunisians Ramzi A and Mohamed Oualid G, a Tunisian named Chokri C, and an Albanian named Artan - and a woman of dual French-Albanian nationality identified as Enkeldja, Mr Molins said. Ramzi had previous convictions for drugs and petty crime.
All were locked up pending further investigation.
People close to Bouhlel said he had shown no signs of radicalisation until very recently. But Mr Molins said information from his phone suggested he could have been preparing an attack as far back as May 2015.
One photo in his phone, taken May 25 2015, was an article on Captagon, a drug said to be used by some jihadis before attacks.
The Islamic State group has claimed responsibility for the attack, though authorities say they have not found signs the extremist group directed it.
The probe, which involves more than 400 investigators, confirmed the attack was premeditated, the prosecutor said. Telephone records were used to link the five to Bouhlel, and allegedly to support roles in the carnage.
Bouhlel and a 30-year-old French-Tunisian with no previous convictions had phoned each other 1,278 times in a year, Mr Molins said.
The prosecutor said a text message from the same man found on a phone seized at Bouhlel's said: "I'm not Charlie; I'm happy. They have brought in the soldiers of Allah to finish the job."
The message was dated three days after the January 2015 massacre at Charlie Hebdo, the satirical publication in Paris, and referred to the worldwide phrase of solidarity for the victims "I'm Charlie".
Hours after the July 14 attack in Nice, the same man filmed the bloody scene on the promenade.
The aftermath of the Nice attack has seen accusations that security was wanting despite the state of emergency in place since the Paris attacks last November.
French officials defended the government's security measures in Nice on the night of the attack, even as the interior minister acknowledged that national police were not, as he had claimed before, stationed at the entrance to the closed-off boulevard.
Bernard Cazeneuve's clarification came after a newspaper accused French authorities of lacking transparency in their handling of the massacre.
Mr Cazeneuve said on Thursday that only local police, who are more lightly armed, were guarding the entrance to the Promenade des Anglais when Bouhlel drove his truck down it.
He then launched an internal police investigation into the handling of the Nice attack.
President Francois Hollande said the conclusions of that investigation will be known next week. He said any police "shortcomings" will be carefully addressed but defended French authorities' actions.
The National Assembly, meanwhile, extended France's state of emergency for six more months.
The security measure had been in place since the November 13 Paris attacks that killed 130 victims and were claimed by IS.