The man behind the Bastille Day truck attack in Nice planned the atrocity for months and was helped by accomplices, French prosecutors have said.
Mohamed Lahouaiej Bouhlel killed 84 people when he drove his truck through crowds gathered to watch a fireworks display on the Promenade des Anglais last week.
Bouhlel was shot dead by police at the scene, but five suspects have since been arrested and are facing preliminary terrorism charges for their alleged roles in helping the driver.
Prosecutor Francois Molins said his office, which oversees terrorism investigations, has information from Bouhlel's phone which shows searches and photos indicating he had been studying an attack since 2015.
The update on the investigation came as French officials rushed to defend the government's security measures amid criticism following the attack.
Interior minister Bernard Cazeneuve acknowledged that national police were not, as he had previously claimed, stationed at the entrance to the Promenade des Anglais, which had been closed to traffic at the time of the attack.
His clarification came as a newspaper accused officials of lacking transparency in their handling of the massacre.
Retreating from previous claims, Mr Cazeneuve said only local police, who are more lightly armed, were guarding the entrance Bouhlel used to access the walkway.
Mr Cazeneuve has now launched an internal police investigation into the handling of the attack.
President Francois Hollande said the conclusions of that investigation will be known next week. He insisted any police "shortcomings" will be carefully addressed, but he defended French authorities' actions.
"There's no room for polemics, there's only room for transparency," he said. "The necessary, serious preparations had been made for the July 14 festivities."
French Prime Minister Manuel Valls joined a chorus of officials defending themselves against the charges, and made a public show of support for Mr Cazeneuve, in whom he said he has full trust.
"Lies debase public debate... the government has nothing to hide," Mr Valls said.
Their comments came after French newspaper Liberation said Mr Cazeneuve lied about the whereabouts of the national police officers and cars, and accused authorities of lacking transparency.
Using witness statements and photos, Liberation claimed only one local police car was stationed at the entrance to the boulevard.
The paper quoted Nice police officer Yves Bergerat, who said the guns and bullets of the local force are not even equipped "to puncture the tyres", let alone shatter the windscreen, of a truck that size.
In a statement, Mr Cazeneuve accused the paper of conspiracy theories and said several "heroic" national police - who killed the attacker after an exchange of fire - were stationed further down the promenade.
The National Assembly has extended France's state of emergency for six months. The security measure has been in place since the attacks in Paris in November which killed 130 people.
