More than five years ago, Al Qaeda called on its followers to use cars as tools of terror. Soon after, the terrorist group that had brought down the twin towers scrapped the idea, feeling that the tactic was too indiscriminate, too likely to kill Muslims.

But the idea of the car as a weapon was revived by the Islamic State shortly after it broke with Al Qaeda in 2014 following years of discord over the Islamic State’s brutal methods.

“If you are not able to find an I.E.D. or a bullet, then single out the disbelieving American, Frenchman or any of their allies,” the group’s spokesman, Abu Muhammad al-Adnani, said in a speech. “Smash his head with a rock, or slaughter him with a knife, or run him over with your car,” he said, according to a translation provided by the SITE Intelligence Group.