U.S. officials say there’s no sign that Islamic State group directed a deadly New Year’s Day attack in New Orleans. But early evidence that the attacker drew inspiration from the group highlights its ability to spur mass violence despite multinational efforts to wipe it out.

Authorities say the suspect in that attack, Shamsud-Din Jabbar, posted videos online pledging support for the group before driving a pickup truck into a crowded French Quarter street during New Year’s celebrations, killing 15 people. Officials said he had an Islamic State (IS) group flag in his rented truck.

The presence of the flag and Jabbar’s support help explain why authorities were quick to label the deadly attack an act of terrorism. While officials have said for years that IS has been largely defeated, researchers say it has inspired more attacks in recent years and concerns are growing about its re-emergence in its former stronghold of Syria.