(CNN)[Breaking news update, posted at 9:25 a.m. ET]

"We have an enemy who is going to continue to strike all the people, all the countries who have freedom as a fundamental value," French President François Hollande said Friday at a prefecture headquarters in Nice after visiting some of the victims of Thursday's terror attack.

He referred to the attack as "an unspeakable act." He added: "Why Nice? Because it's a town famous across the world, (a) beautiful town, one of the most beautiful (in the world.) Why the 14th of July? Because it" was Bastille Day, celebrating freedom.

[Previous story, posted at 9:12 a.m. ET]

The popular Promenade des Anglais in Nice would normally be packed with tourists and residents on a sunny afternoon in July; but Friday, screens blocked off more than a mile of the famous boulevard as authorities removed bodies and evidence after a man in a truck mowed down revelers at the Bastille Day fireworks.

At least 84 people were killed Thursday night, including several children, authorities said, and more than 100 people were injured, some critically.

And again, for the third time in 18 months, a battered France will fly its flags at half-staff to mourn the victims of yet another deadly terrorist attack on its soil.The Mediterranean city, popular with tourists for its beaches, restaurants and general ambiance, was left devastated by the horrific attack.

Authorities covered the dead in the street with blue sheets so 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 that France is having to learn to live with regular attacks.

French President François Hollande said Friday at least 50 people were in critical condition, "between life and death," after he visited some of the victims at a hospital in Nice.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.

Slowly, authorities are putting names to the bodies. Americans Sean Copeland, 51, and his son, Brodie, 11, of Texas were killed in the attack, 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 international police agency Interpol announced on its Twitter account it would deploy a team to Nice "to provide disaster victim identification assistance."

No group has claimed responsibility for the attack, and not enough is yet known about the driver of the truck to determine if the act had anything to do with religion. 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.According to witnesses and a blurry cell phone video, the white truck rolled purposefully toward the crowds just after the fireworks display finished. The video shows the vehicle accelerating as people scattered in front of it and a few people chased it from behind.

The truck plowed through the crowds for more than a mile before police were able to intervene and fatally shoot the driver.

On Friday, authorities began searching an apartment block believed to be the assailant's home, searching for any clue of what may have motivated him.Just hours before the carnage, hundreds, if not thousands, had gathered on the Promenade des Anglais, which was cordoned off, to watch a colorful display of fireworks and live music.But as the last firework fizzled, gunfire rang out -- authorities and witnesses say the driver shot from the cab of the truck -- and the truck accelerated down the crowded street. Among the dead were several children, officials said.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 Hollande had called for three days of mourning, from Saturday to Monday, when public buildings will fly the national flag at half staff.Apart from those confirmed dead, three Australians, two Chinese and one British national were injured, officials said.The country was just about ready to lift its state of emergency, which was put into place in the wake of the November terror attacks in Paris that killed 130 people, the worst attack in the country's history.

The state of emergency would have expired later this month, but Valls said Friday that a bill to extend it would be submitted to parliament by Tuesday.

In a national address in the early hours of the morning, Hollande said the act was, without doubt, a terror attack.

"France is afflicted, but she is strong, and she will always be stronger than the fanatics who want to strike her today," he said.

He added that the choice of the day -- Bastille Day, when France celebrates its post-French Revolution republic -- was particularly poignant.The day is a "symbol of liberty," the President said, noting that "human rights are denied by fanatics and France is quite clearly their target."France had put intense security in place for Euro 2016, the international soccer tournament that just ended. No major attacks occurred during the event.

While the police response to Thursday's attack appeared to be speedy, questions are now being raised about how the man was able to breach security at the event.

Authorities found several fake rifles and fake grenades inside the truck, according to BFM-TV reporter Cecile Ollivier. A 7.65 caliber handgun was found on the attacker who was known to authorities for weapons possession crimes, but nothing terror related, according to Ollivier.

CNN terrorism analyst Paul Cruickshank said "no country in the western world is threatened more by jihadis and terrorism than France."

"This is a big step back here. They are absolutely exhausted after a year and a half of intense efforts to try and protect this country," Cruickshank said.

"The painful reality here is that if it wasn't going to be this promenade, it would have been any other promenade."

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.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.