It was just after 3 a.m. on Wednesday, and while much of the country had already gone to bed after toasting the new year, the party was still going — as it usually does — on Bourbon Street in New Orleans.

Then came the sound of a white pickup truck, accelerating. In an instant the city’s best-known, most popular public space was transformed into a scene of death and terror.

“We heard him punch the gas and then the impact and then the screams,” said Kimberly Stricklin, of Mobile, Ala., who watched with her husband as the truck careened onto Bourbon Street and slammed into the crowd. “It just took a moment to register, it was just so frightening — it was like something out of a horror movie.”

Mrs. Stricklin said she was haunted by the memory of the sounds one victim had made. “I can’t get over that girl’s screams,” she said.

Described by federal authorities as a deliberate act of terror, the attack killed at least 15 people, injured about three dozen others and left New Orleans, a city of 364,000, on edge; investigators said at a news conference on Wednesday that they believed that the driver did not act alone.

The driver, who was killed in a shootout with police, was identified by the F.B.I. as Shamsud-Din Bahar Jabbar, 42, a U.S. citizen and U.S. Army veteran from Texas. He had loaded his rented truck with weapons and at least one “potential” improvised explosive, authorities said, and an Islamic State flag was found on the trailer hitch of his rented white Ford pickup.