Mohamed Lahouaiej Bouhlel, the man behind the deadly Nice lorry attack which killed at least 84 people, was radicalised "very quickly", according to the French prime minister.
Authorities are yet to produce evidence linking the 31-year-old Tunisian, shot dead by police, to Islamic State. However, Manuel Valls said there were no doubts about the assailant's motives.
Following Thursday's attack on people celebrating Bastille Day in the French Riviera city the country's state of emergency was extended by three months.
Speaking with Sunday newspaper Le Journal du Dimanche, Mr Valls said he was convinced of the Islamist nature of the attack, which saw people driven over and shot at.
"The investigation will establish the facts, but we know now that the killer was radicalised very quickly, " Mr Valls said.
"The claim on Saturday morning by Islamic State and the fast radicalisation of the killer confirms the Islamist nature of this attack".
Officials said on Saturday that people questioned by police had indicated that Bouhlel had undergone a rapid transformation from someone with no apparent interest in religion.
Mr Valls, who said security services have prevented 16 attacks over three years, said he believed Islamic State had manipulated an unstable individual to carry out the attack.
"Daesh gives unstable individuals an ideological kit that allows them to make sense of their acts ... this is probably what happened in Nice's case," Mr Valls said.