Split-Second Decisions: How a Supreme Court Case Shaped Modern Policing

The same standard may also make it more difficult to combat racial bias in the use of lethal force, even though Black suspects are more than twice as likely as people of other races to be killed by the police, said Jeffrey Fagan, a law professor at Columbia.

“All an officer has to say is, ‘I feared for my life’ — those are the magic words,” he said. But the statistics strongly suggest that the “police are more likely to form that sense of imminent danger when confronting a Black person than a white person.”

Critics of the standard cite a litany of police killings of innocent Black people. In 2015, Cleveland police officers screeched their cruiser to a halt just a few feet from Tamir Rice, a Black 12-year-old playing in park with a toy replica gun, and within two seconds had shot him because they deemed him a threat. He died the next day.

The next year, a police officer in a suburb of Minnesota pulled over Philando Castille, a 32-year-old Black man driving with his girlfriend and daughter. The officer shot him five times at close range, killing him in the seat of his parked car, out of a mistaken fear that he might have been reaching for a gun.

In 2018, the police raced to a street corner in the Crown Heights section of Brooklyn to investigate reports of a possible gunman, and moments later shot and killed Saheed Vassell, a 34-year-old mentally ill Black man who was well known around the neighborhood. He had been pointing a piece of pipe they mistook for a weapon.

None of the officers faced charges, though the cities where Mr. Rice and Mr. Castile were killed paid millions of dollars to settle civil claims over their deaths.

The split-second standard “has become a way to insulate officers from any critical review,” said Seth Stoughton, a professor at the University of South Carolina School of Law. A former policeman in Tallahassee, Fla., he testified as an expert witness in the trial of Derek Chauvin, the officer who killed Mr. Floyd.