A US citizen with an Islamic State flag and "hellbent" on carnage steered a pick-up truck into a crowd of New Year revellers in New Orleans, killing at least 15 people and wounding dozens, officials said.
The FBI identified the attacker as Shamsud-Din Jabbar, a 42-year-old US citizen from Texas and an army veteran.
He appeared to have been a real estate agent working in Houston and had served as an IT specialist in the military.
Officials said they were searching for accomplices but gave few details.
Police Superintendent Anne Kirkpatrick described Jabbar as a "terrorist", while the FBI said "an ISIS flag was located in the vehicle", using another name for the Islamic State armed group.
"The FBI is working to determine the subject's potential associations and affiliations with terrorist organisations," the agency said in a statement.
Officials said a manhunt was under way, with FBI agent Alethea Duncan warning that authorities "do not believe that Jabbar was solely responsible".
"We're hunting some bad people down," Louisiana Governor Jeff Landry said.
Authorities had earlier put the toll at 10, but an FBI spokesman said it had risen to 15, citing the New Orleans coroner's office.
Aiming for 'carnage'
Police said the incident began at around 3.15am (9.15am Irish time yesterday) in the heart of the French Quarter, which was packed with people celebrating the start of 2025.
The suspect drove a white Ford F-150 electric pickup into a group of pedestrians, then exited and was killed in an exchange of gunfire with police - two of whom were wounded.
Two homemade bombs were found and neutralised, the FBI said.
"This man was trying to run over as many people as he possibly could," Supt Kirkpatrick told reporters.
Driving at "very high speed" and in a "very intentional" manner, "he was hellbent on creating the carnage and the damage that he did," she said.
The wounded officers were in a stable condition and Supt Kirkpatrick said they would recover.
The Pentagon said Jabbar had served in the army as a human resource specialist and an IT specialist from 2007 to 2015, and then in the army reserve until 2020.
He served in Afghanistan from February 2009 until January 2010, an army spokesperson said, adding that he held the rank of staff sergeant at the end of his service.
US President Joe Biden said in a statement that his government "will not tolerate any attack on any of our nation's communities".
Mr Biden said that the suspect posted videos "indicating that he was inspired by ISIS" hours before ramming a truck into the crowd.
"The FBI also reported to me that mere hours before the attack he posted videos on social media indicating that he was inspired by ISIS," Mr Biden said of suspect Jabbar.
President-elect Donald Trump immediately linked the attack to illegal immigration, giving no evidence, in a statement made before authorities made clear the suspect was American.
Mr Trump also claimed that the nation's crime rate "is at a level that nobody has ever seen." In fact, violent crime is sharply down across the country, according to the FBI.
Horror in iconic neighbourhood
In the early hours of the year's first day, revellers were celebrating in the French Quarter, a district renowned for its bars, restaurants, jazz history and Mardi Gras parties.
Bystander Zion Parsons said he saw that instantly turn into a scene of horror.
"The best way I can describe it is truly a war zone," he told CNN. "There were bodies and blood and all the trash."
"People were terrified, running, screaming," he said.
Another witness, Jimmy Cothran, told ABC that the mayhem was "insanity".
"We instantly counted, I'd say, 10 bodies - six clearly graphically deceased, and the others yelling with no one around," he said.
New Orleans is one of the most heavily visited destinations in the United States and on 9 February will stage the NFL's Super Bowl game - one of the biggest sporting events of the year.
The attack came just hours before the city was due to host the Sugar Bowl, a major college football game featuring teams from the University of Georgia and Notre Dame.
That game was delayed for 24 hours, officials said.
Policing had already been heavy over the New Year's holiday, according to the city, as authorities braced for big crowds.
The city police department had announced staffing at "100 percent, with an additional 300 officers assisting from partner law enforcement agencies," including on horseback and using unmarked units.
President Biden said authorities were probing "any possible connection" between the truck-ramming attack in New Orleans and the explosion of a Cybertruck outside a Trump hotel in Las Vegas.
Law enforcement are investigating the explosion of the Tesla vehicle "including whether there's any possible connection with the attack in New Orleans," Mr Biden told reporters, adding: "Thus far, there's nothing to report on that score."
World condemnation
Tánaiste and Minister for Foreign Affairs Micheál Martin paid tribute to those impacted by the attack.
In a statement on X, Mr Martin said he was "shocked and horrified by the suspected terror attack that left ten dead and many more injured in New Orleans".
"Our thoughts and condolences go to all the victims and their families.
"Ireland stands in solidarity with the people of New Orleans and the US at this tragic time," Mr Martin said.
UK Prime Minister Keir Starmer the New Orleans attack as "shockingly violent".
He said his thoughts were with the victims, their loved ones and the wider US.
German Chancellor Olaf Scholz said the attack was "senseless hatred", while French President Emmanuel Macron said "New Orleans, so dear to the hearts of the French, has been struck by terrorism".
NATO Secretary General Mark Rutte said he was "deeply saddened" by the "horrific attack" and Mexico's president Claudia Sheinbaum said the country stood "in solidarity with the families of the victims and with the people of the United States".
