A number of videos from the southern city of Nice, France - some too graphic to share - show bodies strewn across the roadway where people had gathered to see the firework show for French National Day.
French media is now saying the attacker is 31 years old, a French-Tunisian citizen and likely a resident of Nice.
"ISIS has said repeatedly, especially in France, if you can't get a weapon, get a truck," said Ed Turzanski, Foreign Police Research Institute.
Anti-terror experts say it is unprecedented in France, which has some of the toughest gun laws, a vehicle was being used a weapon of mass destruction.
"We need to stop looking at the weapon. It's not the gun, it's the terrorists, and that's where the French are going to bear down, I think very strongly," said Turzanski. "They've been pushing back very hard at Islamists."
At a Bastille Day celebration at the Good King Tavern in South Philadelphia, people were reacting with shock and dismay.
"It's totally atrocious. I still remember the night I caught wind of everything that happened in Paris. I have a lot of friends that still live there," said Chloe Grigris, owner. "Actually my family is from the south of France so the fact that it was so close to home is definitely scary."
Charles Palous, who grew up in Nice, says his mother had just left the area where the firework show was taking place.
"All of my family and friends, as far as I know, are fine, but it's pretty scary," said Palous. "It makes the celebration a little less celebratory."
France has been under a State of Emergency since a previous terror attack late last year. Anti-terror export Ed Turzanski says that's likely to continue indefinitely.
"Monitoring mosques, social media and in the cases where someone behaves in a suspicious way," said Turzanski. "The French are talking about expelling people."
The State of Emergency in France gives law enforcement authorities there greater power to investigate potential terror threats.
Experts, locals react to terror attack in Nice