ISIS claimed in a statement Saturday morning that it was responsible for Thursday's deadly truck attack by Mohamed Lahouaiej Bouhlel, a 31-year-old French-Tunisian citizen and Nice resident who was killed by police during the rampage. It called Bouhlel "a soldier" of its organization.
A French government official suggested that the suspect may have adopted an extremist ideology recently.
The suspect "must have been radicalized very quickly ... according to the elements we have been gathering from those we are questioning," French Interior Minister Bernard Cazaneuve said after a meeting of government ministers Saturday. Earlier, Cazaneuve had told French TV that he could not confirm whether Bouhlel was linked to radical Islam.
Defense Minister Jean-Yves Le Drian also suggested that Bouhlel may have been influenced by ISIS to conduct the attack.
"Daesh (ISIS) doesn’t organize but inspire," he said.
"We are confronted by people who are committing violent acts without even being trained or having any weapons," Le Drian said, adding that the country needs to rethink its defense strategy against such attacks.
Authorities in France have detained five people for questioning, including the ex-wife of the suspect and four acquaintances or friends.
Bouhlel's father meanwhile was interviewed from Tunisia on French radio. He said his son, whom he had not seen in four years, had psychiatric problems.
The suspect's brother, interviewed on Tunisian radio Al Jawhara FM, said that to his knowledge Mohamed was not a practicing Muslim.
The defense minister also said that in response to the attack France will maintain its mobilization of 10,000 additional troops.
In addition to killing at least 84 people on the Promenade des Anglais in Nice, Bouhlel injured 303 others, including 30 children, from infants to teens, according to French health officials.
A Nice City Hall spokesman told ABC News there were about 30,000 people at the idyllic seafront promenade on Thursday night. Families with young children gathered along its waterfront to watch a fireworks display for Bastille Day, France's national holiday.
Out of the hundreds initially hospitalized, 121 are still receiving treatment and 26 of them are in intensive care, including five children. One of them is a 7-year-old boy from Romania, officials said. It was not immediately clear whether health authorities had located his family.
The youngest child injured is a 6-month-old baby, officials said.
The Promenade des Anglais, one of Europe's most famous streets, reopened partially to one-way traffic today. The palm-lined promenade will reopen fully on Monday after a national minute of silence at noon local time, the Mayor of Nice's office told French media.
ABC News' Morgan Winsor, David Caplan and Louise Dewast contributed to this report.