The suspect, who was shot dead at the scene after firing at police, has been identified as Shamsud-Din Jabbar - a 42-year-old Texan who once served in Afghanistan.
He drove from Houston to New Orleans on December 31 and posted five videos on Facebook between 1.29am and 3.02am on the morning of the attack in which he said he supported IS, the Islamic militant group with fighters in Iraq and Syria, the FBI said.
In the first video, Jabbar explains he had previously planned to harm his family and friends but was concerned that the media coverage would not focus on the "war between the believers and the disbelievers," FBI deputy assistant director Christopher Raia said at a press conference.
Authorities in other US cities say they have boosted security as residents of New Orleans mourned. (AP PHOTO)
Jabbar also said in the videos that he had joined IS before last summer and provided his last will and testament, Raia said.
"This was an act of terrorism," Raia said.
"It was premeditated and an evil act."
The FBI was still investigating Jabbar's "path to radicalisation" but the evidence reviewed so far showed that he was clearly inspired by IS, Raia said.
Surveillance video footage showed Jabbar placing two improvised explosive devices in coolers a few hours before the attack at intersections around Bourbon Street, the popular New Orleans tourist destination where the attack unfolded, Raia said.
They were both rendered safe at the scene.
Other people were seen on video looking at the coolers, who investigators now believe were just curious passers-by and not accomplices.
Abdur-Rahim Jabbar, Jabbar's younger brother, told the Associated Press on Thursday that it "doesn't feel real" that his brother could have done this.
"I never would have thought it'd be him," he said.
"It's completely unlike him."
He said that his brother had been isolated in the last few years but that he had also been in touch with him and he did not see any signs of radicalisation.
"It's completely contradictory to who he was and how his family and his friends know him," he said.
The FBI said there appeared to be no link between the attack in New Orleans and the episode in Las Vegas on the same day in which a Tesla Cybertruck packed with petrol canisters and large firework mortars exploded in flames outside the Trump International Hotel in Las Vegas, just weeks before president-elect Donald Trump returns to the White House on January 20.
The injured victims in the New Orleans attack included two police officers wounded by gunfire from the suspect, taking place a mere three hours into the new year on Bourbon Street in the historical French Quarter.
At least 15 people were killed, including the suspect, the FBI said.
Among the victims were the mother of a four-year-old who had just moved into a new apartment after getting a promotion at work, a New York financial employee and accomplished student-athlete who was visiting home for the holidays and an 18-year-old aspiring nurse from Mississippi.
Meanwhile, authorities in other US cities said they had boosted security, including at Trump Tower and Times Square in New York City, adding that there were no immediate threats.
The FBI said an IS flag was found on the trailer hitch of the rented vehicle involved in the New Orleans attack.
US President Joe Biden condemned what he called a "despicable" act.
Public records showed Jabbar worked in real estate in Houston.
In a promotional video posted four years ago, Jabbar described himself as born and raised in Beaumont, a city about 130km east of Houston.
Jabbar was in the regular army from March 2007 until January 2015 and then in the army reserve from January 2015 until July 2020, an army spokesperson said.
He deployed to Afghanistan from February 2009 to January 2010 and held the rank of staff sergeant at the end of service.
