BREAKING NEWS –  At least 84 people were killed and several others were seriously injured Thursday when a suspected terrorist drove a large truck into a crowd that had gathered for a Bastille Day fireworks display in the southern French city of Nice.

There was no immediate claim of responsibility, but two sources, including a U.S. counterterrorism source who collects and monitors jihadist social media, told Fox News that accounts linked to ISIS were “celebratory” and their followers were told to use the hashtag “Nice.”

Initial reports said the man was driving a truck loaded with guns and hand grenades. However, a security source told Agence France-Presse (AFP) that an “inactive” grenade was found as well as “several fake rifles.”

The death toll was confirmed by French Interior Minister Bernard Cazeneuve early Friday morning. A source told Fox News that two Americans – a father and his son – were among the dead in the attack.

Regional President Christian Estrosi also said that more than 10 children were among the dead.

Earlier, French President Francois Hollande said that children were among the dead, and said his country was "under the threat of Islamic terrorism. We have to demonstrate absolute vigilance and show determination that is unfailing."

Hollande also announced that he would extend France's state of emergency by another three months, until Oct. 26. France has been on its highest state of alert since ISIS terrorists killed 130 people in Paris this past Nov. 13.

French Interior Ministry spokesman Pierre-Henry Brandet said the truck driver was killed by police. According to AFP, the man fired his gun several times into the massive crowd of Bastille Day revelers before being shot dead. Images showed the truck riddled with bullets on its windshield.

The Paris prosecutor's office announced that it was opening an anti-terrorism investigation into the attack.

Sky News and the Nice-Matin newspaper reported the driver was a 31-year-old Nice resident of Tunisian origin. According to BFM-TV in France, the man was known to police but not for terrorism. No other details were immediately available.

Estrosi said some of the city's 1,200 security cameras had pinpointed the moment the attacker boarded the truck, far from the seaside "in the hills of Nice" and could follow his path to the promenade. Estrosi called for the investigation to focus on any accomplices.

A U.S. official told Fox News that the attack was in line with ISIS, which has become "increasingly brazen" in its attacks as it comes under increasing military pressure in Iraq and Syria.

A 2010 edition of Al Qaeda in the Arabian Peninsula's propaganda magazine "Inspire" also called on would-be terrorists to "use a pickup truck as a mowing machine ... to mow down the enemies of Allah."

The horror began at around 10:30 p.m. local time. A local government official told Fox News that the truck was driving full speed when it hit the crowd on the Promenade des Anglais, a major boulevard along the Mediterranean Sea.

Damien Allemand, a journalist for Nice-Matin, said the fireworks display was over and people were getting up to leave when they heard a loud noise and screams.

"A fraction of a second later, an enormous white truck came along at a crazy speed, turning the wheel to mow down the maximum number of people," he said. "I saw bodies flying like bowling pins along its route. Heard noises, cries that I will never forget."

Allemand said people took shelter in a nearby restaurant, where he continued to hear people shouting for missing family members. He ventured out and saw bodies, blood and body parts all along the road.

"This evening, it was horror," Allemand concluded.

Nice-Matin posted an image of the truck on Twitter, captioning it, "The truck that drove into the crowd."

Another witness, Wassim Bouhlel, told the Associated Press that he saw the truck drive into the crowd, then witnessed the driver emerge with a gun and start shooting.

"There was carnage on the road," Bouhlel said. "Bodies everywhere."

Video showed men and women -- one or two pushing strollers -- racing to get away from the scenes. And, in what appeared to be evidence of a gun battle, photos showed a truck with at least half a dozen bullet holes punched through its windshield.

Graphic footage showed a scene of horror up and down the Promenade, with broken bodies splayed out on the asphalt, some of them piled near one another, others bleeding out onto the roadway or twisted into unnatural shapes.

"Help my mother, please!" one person yells out amid a cacophony of screaming and crying. A pink girl's bicycle is briefly seen overturned by the side of the road.

Nice's public prosecutor told reporters early Friday that bodies of the victims were scattered for over a mile.

Another witness, identified only as Chloe, told FranceTV Info, "We heard gunfire, a lot. A crowd came to us and told us to run ... We went into a tapas bar and hid in the bathroom for half an hour.

"We went out and again we heard people running and saying 'truck, truck,' so we hid in the bathroom for an hour. The manager took us out by the back door and then I went home."

In addition to extending the country’s state of emergency and the Sentinel operation with 10,000 soldiers on patrol, Hollande said he was calling up "operational reserves," those who have served in the past and will be brought in to help police, particularly at French borders.

He reiterated that France is also bolstering its presence in Iraq and Syria, where he said earlier that military advisers would be on the ground to help Iraqis take back the Islamic State stronghold of Mosul.

President Barack Obama said he condemned "what appears to be a horrific terrorist attack" in the strongest terms.

"On this Bastille Day," Obama said, "we are reminded of the extraordinary resilience and democratic values that have made France an inspiration to the entire world, and we know that the character of the French Republic will endure long after this devastating and tragic loss of life."

Secretary of State John Kerry called the tragedy "horrendous" and said it was "an attack against innocent people on a day that celebrates Liberty, Equality, and Fraternity," a reference to France's national motto. Kerry also asked U.S. citizens in Nice to directly contact friends and relatives to inform them of their well-being.

Presumptive Republican presidential nominee Donald Trump announced late Thursday that he would postpone the announcement of his running mate, which had been scheduled to take place Friday morning.

France's ambassador to the United States, Gerard Araud, characterized the events in Nice as a "terrorist attack."

"Our democracies -- France, the United States, our other partners, we are besieged, we face a terrible threat," Araud said at a Bastille Day reception at the French Embassy in Washington late Thursday.

Flags were lowered to half-staff in Nice and in Paris. Marseille also canceled their fireworks celebrations for Friday.

July 14 is a national holiday in France that commemorates the storming of the Bastille prison in Paris and the start of the French Revolution.