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Louis Scarcella, the former homicide detective, needed paperwork to refresh his memory and glasses borrowed from a prosecutor to read documents he was handed, including reports that he had written himself more than 20 years ago.

But as he looked out from the witness box in a Brooklyn courtroom on Friday, he could plainly see the row of angry men staring him down in the audience, all wearing hats that said “Wrongfully Convicted.” They were men he had helped to arrest and put in prison decades ago and who had later been exonerated.

Mr. Scarcella was in court to testify about yet another imprisoned man, Nelson Cruz, who claimed he had been framed for murder. Once again, Mr. Scarcella was asked to defend his work, and once again, he did so without apology.

When Mr. Cruz’s lawyer, Justin Bonus, asked him, “Do you stand by all of the investigations you’ve conducted?” Mr. Scarcella replied, “110 percent.”