now-infamous traffic stop of a uniformed Army medic who was held at gunpoint and doused with pepper spray by the police in Windsor, Va., a rural town near Norfolk. The encounter, which occurred in December, was brought to light this month after Caron Nazario, a second lieutenant in the U.S. Army Medical Corps, filed a federal lawsuit.

Body camera footage shows members of the Windsor Police Department threatening and attacking Lieutenant Nazario, who is Black and Latino, after stopping him because he had not yet put permanent license plates on his new Chevrolet Tahoe.

The footage underscores the extent to which police culture has resisted change in much of the country, Dr. Stinson said.

“We only know about this one because he has a lawyer, they filed a civil lawsuit and they were able to get recordings they could release,” he said.

For many victims of police violence and their families, however, there is no video evidence to rely on.

Daly City, Calif., police officers were not wearing body cameras when they got into a struggle with Roger Allen, 44, as he sat in a car idled with a flat tire on April 7. The officers say that Mr. Allen had what appeared to be a gun on his lap, according to Stephen M. Wagstaffe, the San Mateo County district attorney, who is investigating the case. It turned out to be a pellet gun, but an officer fired a fatal bullet to Mr. Allen’s chest during the fracas.

Now Talika Fletcher, 30, said she was struggling to come to terms with the fact that her older brother, who was like a father figure, had joined the grim tally of Black men who died at the hands of law enforcement.

“I never thought in a million years that my brother would be a hashtag,” she said.

She has little faith that the dynamic between Black men and law enforcement will be any better once her 14-month-old son, Prince, grows up.

“The cycle,” she said, “it’s not going change.”

Alain Delaquérière contributed research.

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