As early as 1995, a Department of Justice bulletin on “positional asphyxia” quoted the New York Police Department’s guidelines on preventing deaths in custody. “As soon as the subject is handcuffed, get him off his stomach. Turn him on his side or place him in a seated position.”

As Michael Baden, a former chief medical examiner of New York City, told The Times: “Obese people especially, lying face down, prone, are unable to breathe when enough pressure is put on their back. The pressure prevents the diaphragm from going up and down, and he can’t inhale and exhale.”

Which is exactly what Mr. Garner was trying to tell the officers who were on top of him.

Mr. Garner’s death recalls a similar tragedy involving a less familiar name: Robert Ethan Saylor, a 26-year-old man with Down syndrome who was killed last year in a struggle with three off-duty county sheriff’s deputies at a movie theater in Frederick County, Md. Mr. Saylor was overweight. The officers who killed him were just as inept as Officer Pantaleo and his gang, though with one key difference: When they realized that Mr. Saylor was in distress, they tried to save him. Still, their efforts came too late, because mere moments in a facedown arrest can be deadly.

The Garner killing must lead to major changes in policy, particularly in the use of “broken windows” policing — a strategy in which Officer Pantaleo specialized, according to a report in September by WNYC, which found that he had made hundreds of arrests since joining the force in 2007, leading to at least 259 criminal cases, all but a fraction of those involving petty offenses. The department must find a better way to keep communities safe than aggressively hounding the sellers of loose cigarettes.

And while defenders of the police like to point to thousands of nonfatal misdemeanor arrests as evidence that officers are acting in a way that is reasonable and safe, there can never be a justification for any lethal assault on an unarmed man, no justification for brutality.