A knife attack that injured three people on Saturday in a London subway station was being investigated as a “terrorist incident,” the police said. But the words of a bystander who shouted at the suspect as officers held him down are what have captured public attention.

“You ain’t no Muslim, bruv,” a man can be heard calling out to the suspect on videos of the arrest posted on YouTube. “You’re no Muslim, bruv. You ain’t no Muslim.” “Bruv” is London slang, similar to “bro” in the United States.