Brianna Villafane was in Lower Manhattan protesting police violence in the summer of 2020, when officers charged into the crowd. One of them gripped her hair and yanked her to the ground.

“I felt someone on top of me and it was hard to breathe,” she said. “I felt like I was being crushed.”

The New York City civilian oversight agency that examines allegations of police abuse investigated and concluded that the officer had engaged in such serious misconduct that it could constitute a crime.

Ms. Villafane received a letter from the oversight agency about its conclusions. “I was happy and I was relieved,” she recalled. The next step would be a disciplinary trial overseen by the New York Police Department, during which prosecutors from the oversight agency would present evidence and question the officer in a public forum.

Then last fall, the police commissioner intervened.

Exercising a little-known authority called “retention,” the commissioner, Edward Caban, ensured the case would never go to trial.

Instead, Mr. Caban reached his own conclusion in private.

He decided that it “would be detrimental to the Police Department’s disciplinary process” to pursue administrative charges against the officer, Gerard Dowling, according to a letter the department sent to the oversight agency. The force that the officer used against Ms. Villafane was “reasonable and necessary.” The commissioner ordered no discipline.