Tracing the New Orleans Attacker’s Secret Radicalization

His electric truck was already headed toward New Orleans, traveling from his trailer home outside Houston and past the twinkling oil refineries to the east, when Shamsud-Din Jabbar began capturing a video on his phone in the dark.

“I wanted to record this message for my family,” Mr. Jabbar said. “I wanted you to know that I joined ISIS earlier this year.”

Mr. Jabbar then added a chilling addendum.

“I don’t want you to think I spared you willingly,” he said, according to details of the video reviewed by The New York Times. He told his family that he had previously conceived of organizing a “celebration” for them and then making everyone “witness the killing of the apostates.”

The words were among Mr. Jabbar’s last before he plowed his rented pickup truck through early morning New Year’s crowds on Bourbon Street in New Orleans, killing 14 people before dying in a shootout with the police. He carried with him in the truck the flag of the Islamic State terrorist group, better known as ISIS.

The devastating violence revealed a secret radicalization that confounded his loved ones, who knew him as a decorated Army veteran who held a $120,000-a-year job as a “senior solutions specialist” focused on government and public services at the international accounting firm Deloitte. Days later, investigators were still trying to trace exactly how Mr. Jabbar had managed to descend into such a murderous state without detection.