NICE, France -- France will once again fly its flags at half-staff to mourn the victims of a deadly terrorist attack on its soil.
At least 84 people were killed in Nice Thursday night when an apparent lone driver of a truck opened fire on a crowd and fatally mowed down people celebrating Bastille Day, the French national holiday.
Two victims were identified on Friday as Americans Sean Copeland, 51, and his son, Brodie, 11, from Texas, the Austin American-Statesman reported, citing a statement from the family.
U.S. officials confirmed that at least two Americans were killed in the attack, but did not name them.
The Mediterranean city, popular with tourists for its beaches, was left devastated by the horrific attack, as authorities covered the dead in the street with blue tarpaulins so that emergency vehicles could find them for evacuation.
"France has been struck once again in her flesh, on the 14th of July, on the day of our national celebration," French Prime Minister Manuel Valls said Friday.
The attacker wanted to "harm the very idea of national unity," he said, adding solemnly that France is having to learn to live with regular attacks.
Children among the dead
Just hours before the carnage, hundreds, if not thousands, had gathered on the famous Promenade des Anglais, one of the city's main thoroughfares, in a colorful display of fireworks and live music.
But as the last firework fizzled, the gunfire rang out and the white truck accelerated down the crowded street. Among the dead were several children, officials said. At least 18 people remain in critical condition and the death toll grew through the night.
Although the road was cordoned off, the driver managed to cover more than a mile along the packed waterfront strip before being shot by police.
Andy McArdy told CNN he saw the truck driving at high speed along the promenade and the driver "was firing a machine gun while driving."
He said everyone ran, many into a restaurant. "They didn't know where to go, they were looking for an exit -- they were hoping they'd find an exit out the back. They had to stay there for a couple of hours, but people wouldn't even come out -- they were so frightened -- until the police came and said it was OK to come out," he said.
Valls said French President François Hollande had called for three days of mourning, from Saturday to Monday.
Apart from the Americans confirmed dead, three Australians were injured, Australian Foreign Minister Julie Bishop said, and British Foreign Secretary Boris Johnson said a UK national was also injured. Two Chinese citizens were injured as well, the Chinese consulate in Marseille told CNN.
Attack on France's 'symbol of liberty'
In a national address in the early hours of the morning, Hollande said the act was, without doubt, a terror attack.
He added that the choice of the day -- Bastille Day, when France celebrates its post-French Revolution republic -- was particularly poignant.
He said that the day is a "symbol of liberty," and that "human rights are denied by fanatics and France is quite clearly their target."
Hollande and Valls arrived in Nice on Friday after a meeting of the Security and Defense Council.
Valls told reporters Friday after the meeting that a bill to extend an existing state of emergency would be submitted to parliament by Tuesday.
The measure was in place in the wake of the terror attacks in Paris in November, the worst terror attack in the country's history, which left 130 people dead. The state of emergency was due to expire later this month.
"France is afflicted, but she is strong, and she will always be stronger than the fanatics who want to strike her today," he said.
So far, no group has claimed responsibility for Thursday's attack.
Witnesses describe horror
Eric Dartell was eating at a restaurant on the street where the incident happened. "You can see wreckage all along the way: a body, bicycles, street lamps and debris everywhere," he said.
American Dominique Molina, who was watching from a balcony, said the fireworks had just ended and the crowd on the beach was dispersing.
"People were flooding the streets, just walking away from the show, and I heard a lot of loud noises and people were screaming and so to the west, a big moving truck was driving on the promenade, just barreling over people and hitting -- running people over."
She estimated the truck moved at 20 to 25 mph. Molina said her teenage son witnessed the carnage. "It's something you're not supposed to see," she said. "I grabbed my son, I felt like shielding him, protecting him from seeing that. It happened so fast, it was like in slow motion."
Paul Delane, another American, described the chaos. "All of a sudden, just people, thousands of people, started running in one direction.
"My partner took my hand immediately and we started running with everybody and honestly in my head I had no idea what was going on and the music was so loud and I didn't really see a truck, but just people running and screaming and crying and people carrying their children, and it was just very frightening."
Facebook has activated the Safety Check feature for people in and around the area of the attack.
A tourist from Dallas, Kristen Crouch, lamented the climate of violence that spans the globe, from her hometown to the French city, which she was visiting for a friend's wedding.
"It's really sad when you've been marked safe twice on Facebook in the last week. We shouldn't live in a world like that," she said.
"We quickly got back to the hotel and locked the door."
One man came out of the Pasteur Hospital in Nice, limping with an ankle injury and with blood on his feet and shorts. He said he worked in a restaurant near the scene of the attack.
"Two kilometers. He just kept driving, straight through. I work there. And my daughter. My daughter. I don't know," he said, still in a state of shock.
Investigation under way
CNN terrorism analyst Paul Cruickshank said "no country in the western world is threatened more by jihadis and terrorism than France."
Anti-terror prosecutors have taken over the investigation, according to BFM-TV, citing the prosecutor's office.
Authorities are attempting to determine whether the identity card of a 31-year-old French-Tunisian found in the truck cab matches the body of the driver in the attack, according to a police source cited by AFP. The driver was a Nice resident, according to the Nice Matin newspaper.
Interior Minister Bernard Cazeneuve said he was deploying 70 police, medical and technical specialists in order to make sure that the remains of those killed were quickly returned to the families. Hospitals in the city have launched an urgent appeal for blood donors.
France had put intense security in place for Euro 2016, the international soccer tournament that just ended. No major attacks occurred during the event.
A 7.65 caliber handgun was found on the attacker, and authorities also found several fake rifles and fake grenades inside the truck, according to BFM-TV.
A source close to the investigation tells CNN that the assailant was known to authorities for petty crime violations, based on a provisional identification of the attacker. French authorities had not opened a counter-terrorism surveillance file on him and he was not known to them for jihadism or Islamist extremism.
Leaders around the world have denounced the brutal incident.
U.S. President Barack Obama issued a statement saying, "We stand in solidarity and partnership with France, our oldest ally, as they respond to and recover from this attack."
The presumptive nominees for the U.S. presidential election also reacted to the attack, taking strikingly different tones.
Republican Donald Trump said he'd ask for a declaration of war against ISIS while the Democrat candidate-in-waiting, Hillary Clinton, called for greater intelligence gathering to fight terror groups.
The United Nations condemned the "barbaric and cowardly" terror attack.
The U.S. Consulate in Marseille advised U.S. citizens in Nice to call family and friends to notify them that they are safe. The consulate said it was working with authorities to determine whether any U.S. citizens were injured.