Dozens of similar incidents have occurred across the United States in recent weeks, although it is difficult to assess which attacks are premeditated and which are prompted by rage when drivers find their route blocked by crowds. The tactic has previously been mostly used by extremist jihadist groups like ISIS and Al Qaeda, as well as Palestinian militants.

“It is not just an extremist thing here, but there are social media circles online where people are sharing these and joking about them because they disagree with the protests and their methods,” said Ari E. Weil, the deputy research director at the Chicago Project on Security and Threats of the University of Chicago. “Sharing memes and joking about running over people can lead to real danger.”

There have been at least 66 car attacks nationwide since George Floyd was killed by the Minneapolis police on May 25, Mr. Weil said.