2 Americans Killed in 'Monstrous' Attack in Nice; Witnesses Describe the Mayhem

Two Americans were among the more than 80 killed in an attack in Nice, France, Thursday night, when a driver plowed into a large crowd with a truck hauling grenades and other weapons during Bastille Day celebrations, the U.S. State Department said.

Officials did not name the two Americans, although family members confirmed their identities to ABC News.

"We are aware that two U.S. citizens were killed in the attack. We are working with local authorities to determine if other U.S. citizens were injured in the event," according to a statement released by the State Department this morning. "We strongly urge U.S. citizens in Nice to be in direct contact with your family members in the United States and elsewhere to advise them of your safety."

Sean Copeland and his 11-year-old son Brodie, who were in Nice on a family vacation, were killed in the attack, Alyssa Weaver, the elder Copeland's niece, wrote on Twitter. The Copelands are from Lakeway, Texas, about 30 miles west of Austin.

Weaver tweeted, "I am asking for prayers for my family, due to the tragedy in Nice, France I lost my uncle Sean and my 11 year old cousin Brodie I ask for prayers for my aunt Kim and my two other cousins Maegan and Austin during this tough time and prayers for my family’s safe trip back home #CopelandStrong”

The death toll climbed overnight from 80 to 84 people, including several children, with dozens more were injured and receiving treatment, according to the French Ministry of the Interior.

Witnesses described how a festive night watching fireworks on the seafront for the French holiday ended in horror.

Shortly after the fireworks ended, Inga Romanoff was walking down the promenade when she heard gunshots.

"I started running and I looked back and I saw the truck going through the people right in the place where I was standing. It was just seconds," Romanoff said on "Good Morning America." "I don't think people had any time to jump out of the way."

Chaos erupted as people tried to flee the driver's deadly path, Romanoff said.

A woman next to her stumbled and fell, she said. "People kept running over her, and I was trying to pick her up so she could also run."

It was complete chaos," she said. "People were running in different directions."

Sam Helwani, a French teacher from New Jersey, was with friends further from the scene when they saw a crowd running in their direction.

"It was like a mass exodus of people running towards us," Helwani told ABC News today. "I had asked a woman in French, what’s the problem? What’s going on? And she said there’s a man with a gun."

Helwani, who has traveled to Nice for the past six years, said he's still shaken up by what happened, but won't let it impact the rest of his trip.

"I still can't eat anything. I'm still a little shaky," he said. "I'm here until the end of August so I have a really long trip planned and I'm not going to stop doing what I do."

He added: "It’s still the same Nice that I know and love, just trying to recover after reeling from that attack yesterday."

French President Francois Hollande extended the country's state of emergency for three months and was mobilizing reservists. He said it was unclear whether the attacker had any accomplices.

"France is horrified by what has just occurred -- a monstrous act of using a truck to intentionally kill dozens of people celebrating 14th of July," Hollande said during a nationally televised address early today. "France is strong. France will always be stronger than the fanatics who want to strike France today."

Officials have not yet identified the attacker, and there have been no claims of responsibility. The Paris anti-terrorism prosecutor's office was put in charge of the investigation.

Graphic images surfaced on social media showing the mayhem, including the truck barreling through crowds as dozens are seen running from the scene.

The Vatican released a statement this morning condemning the attack.

"Throughout the night we have followed with great concern the terrible news from Nice. On behalf of Pope Francis, we join in solidarity with the suffering of the victims and of the entire French people this day that should have been a great holiday," Vatican spokesman the Rev. Federico Lombardi said in a statement. "We condemn in the strongest way every demonstration of senseless violence, of hatred, terrorism and any attack against peace."

Pope Francis also tweeted this morning that he was praying for the victims.

President Obama also issued a statement calling the incident a "horrific terrorist attack."

Secretary of State John Kerry, who joined French leaders Thursday at Bastille Day celebrations in Paris, said he would continue to stand "firmly" with the French people during this time.

French Prime Minister Manuel Valls said the government is declaring three days of national mourning following the attack in Nice.

Vallas said the national mourning would begin Saturday.