A US army veteran motivated by loyalty to the jihadist group Islamic State likely acted alone when he killed and injured dozens in a truck attack against a New Orleans crowd of New Year revellers, the FBI said Thursday.

Despite initial concerns that Shamsud-Din Jabbar had accomplices still on the run, preliminary investigations show he likely was alone, FBI deputy assistant director Christopher Raia said.

“We do not assess at this point that anyone else was involved,” Raia said.

However, new evidence emerged detailing the extent of the US citizen’s loyalty to Islamic State and his plans to cause mayhem in the attack, which killed 14 and injured more than 30 in the French Quarter district, ending only after he was shot by police.

• SOUTH KOREA: Police raid Jeju Air, airport over fatal crash
• IRAN: Gov’t lifts WhatsApp ban

“He was 100 per cent inspired by ISIS,” Raia said, using an alternative name for the international jihadist group.

Just before the attack, in which 42-year-old Jabbar slammed a rented Ford F-150 pickup into the crowd, he “posted several videos to an online platform proclaiming his support for ISIS,” Raia said.

He also carried a black ISIS flag on the back of the vehicle.

In one video, Jabbar “explains he originally planned to harm his family and friends but was concerned the news headlines would not focus on the ‘war between the believers and the disbelievers.’”

Raia said that Jabbar had planted two homemade bombs in drinks coolers in French Quarter streets. The bombs were viable but were made safe in time, he said.

Raia clarified that the total death toll of 15 from Wednesday’s carnage included 14 victims and Jabbar himself, who died after wounding two police officers in an exchange of gunfire.

The New Orleans attack coincided with a high-profile incident in Las Vegas a few hours later where a Tesla Cybertruck blew up outside a hotel owned by US President-elect Donald Trump.

One person was killed and seven were injured in the incident. It remains unclear whether it was accidental or deliberate.

US media quoted law enforcement sources as saying the person who rented the Telsa truck was a current US Army special forces member.

In another echo of the New Orleans incident, the vehicles in both cases had been rented through the car-sharing app Turo.

Raia said: “At this point, there is no definitive link between the attack here in New Orleans and the one in Las Vegas.”