Even as outrage has mounted over deaths at the hands of the police, it remains notoriously difficult in the United States to hold officers accountable, in part because of the political clout of police unions, the reluctance of investigators, prosecutors and juries to second-guess an officer’s split-second decision and the wide latitude the law gives police officers to use force.

Police departments themselves have often resisted civilian review or dragged their feet when it comes to overhauling officer disciplinary practices. And even change-oriented police chiefs in cities like Baltimore and Philadelphia — which over the last few years have been the sites of high-profile deaths of black men by white officers — have struggled to punish or remove bad actors…

Teresa Nelson, legal director for the A.C.L.U. of Minnesota, said attempts by the city’s police leaders to reform the department’s culture have been undermined by Mr. Kroll, who she said downplays complaints and works to reinstate officers who are fired, no matter the reason.

She said that in a 2015 meeting after a fatal police shooting, Mr. Kroll told her that he views community complaints like fouls in basketball. “He told me, ‘If you’re not getting any fouls, you’re not working hard enough,’” she said.