HARI SREENIVASAN: France began three days of national mourning today for the victims of Thursday's terrorist truck attack in the Riviera resort city of Nice. Today, ISIS claimed responsibility saying the Tunisian man who drove the large truck was one of its soldiers, heeding its call to kill citizens of France and other countries fighting the militant group.

But french authorities have not confirmed the link. The death toll stands at 84… after the truck ran over pedestrians for a mile-and-a-half along Nice's promenade following Bastille day fireworks. Of more than 200 other people who were injured, about 50 remain hospitalized in critical condition.

Special correspondent Jane Ferguson has more from nice.

JANE FERGUSON: Signs of the horror are beginning to fade from Nice's Promenade des Anges.

Parts of this route along the French Riviera have re-opened, as tourists and locals cautiously return to a street known for its glamour, history, and now tragedy. There are more police around, and many have gathered to mourn the dead.

Just to give you some perspective – now that the street has been re-opened – many are coming down to lay flowers and candles at the various spots where people were killed. So all down this street you can see small piles of flowers were each life was lost.

Nice has drawn visitors for centuries – its old world style and beauty are still unmistakable.

That's why Philip Frayne and his wife came here for a holiday. He works at the U.S. embassy and lives in Paris. They were on the promenade just a few minutes before the attacker struck. His wife wanted to stay and dance at the street party, but he wanted to go for dinner.

PHILIP FRAYNE: Had I taken my wife's wishes and stayed to dance at the time we probably would have been right in the crossfire, but we decided to go get a bite to eat first, come back to dance later. That may have saved us, we don't know.

JANE FERGUSON: From a nearby restaurant they heard the attack.

PHILIP FRAYNE: Once gunshots were heard then we all knew we had to get inside and we had to remain low and there were a lot of people amongst us who were crying and who were very upset.

JANE FERGUSON: Other Americans caught up in the panic are still missing.

20-year-old Berkeley student Nicolas Leslie was at the scene of the attack and hasn't been seen since. His family is desperately trying to find him.

A California family is turning to a Facebook page — SOS Nice — to help locate writer Renee Lo Iacono. She was in Nice Thursday night, but her family has not heard from her.

The attacker – shot dead by police – 31 year old Tunisian Mohamed Lahouaiej Bouhel – was not on any French terror lists or known to police as a radical.

The French interior minister said today he apparently "radicalized very quickly." Police here in France have arrested five people including Bouhel's estranged wife. They're still trying to find out if they attacker acted alone or had help with other individuals. Hari.

HARI: On this ISIS claim of responsibility Jane, how are investigators going to know whether the group planned this attack or trained Bouhel?

JANE FERGUSON: They certainly don't know yet Hari, they're trying to look into whether or not this was a coordinated attack with ISIS. But what is clear is through their message. Their statement that they made today that Bouhel had followed their call to attack the citizens of countries that were attacking ISIS.

That is very clearly not the same as saying that they coordinated the attack. Saying That they themselves planned it out. For instance like the Paris attack in in November, which was clearly coordinated and very much so planned inside Syria.

This attack it seemed may have simply been inspired by ISIS, which of course is a very different thing but just as deadly.

HARI SREENIVASAN: Jane Ferguson joining us from Nice, France tonight. Jane thank you very much.