The Islamic State’s magazine, Rumiyah, provided detailed guidance on truck attacks in an issue published in November 2016. The group encouraged supporters to keep driving the car for as long as possible. “To ensure the most carnage over upon the enemies of Allah, it is imperative that one does not exit his vehicle during the attack. Rather, he should remain inside, driving over the already harvested kuffar” — infidels — “and continue crushing their remains until it becomes physically impossible to continue by vehicle.”

In the New York attack, the suspect drove over people in the bike lane until he crashed into a school bus.

According to the magazine, the attacker is expected to jump out and use a secondary weapon, like a gun or a knife. The New York attacker did just that: He jumped out with a pellet gun, though he did not appear to have hurt anyone with it. His aim, though, may have also been to use a knife. The complaint states that a black bag containing three knives was found near Mr. Saipov after he was shot.

Among the more obscure instructions, however, is the manner in which the assailant is supposed to tell the public of his allegiance to the Islamic State. The article states that an assailant should write a note on several sheets of paper, throwing them out the window of the vehicle as the attack is being carried out. The magazine suggests including the phrase, “The Islamic State will remain,” a rendering of the group’s baqiya slogan.

The New York attacker’s papers were found about 10 feet from the driver’s side of the car, the criminal complaint said.

Addressing the ISIS Leader