In a similar fashion, in 2015, Ken Thompson, then the district attorney, held a news conference announcing that he planned to dismiss the conviction of Derrick Hamilton, who spent two decades in prison for a murder that Mr. Scarcella helped investigate. Though Mr. Hamilton has long maintained that Mr. Scarcella framed him, Mr. Thompson, who died last year of cancer, did not quite agree. While he acknowledged to reporters that the sole eyewitness in the case had given “patently false” testimony, as Mr. Hamilton claimed, when it came to Mr. Scarcella, Mr. Thompson said, “We can’t say he acted inappropriately.”

Officials in the district attorney’s office said that in each case that was dismissed, it was for reasons other than the former detective’s police work. Though some defense lawyers have accused the office of “defending” Mr. Scarcella, prosecutors disputed that characterization.

“I think it’s unfair to argue that just because we’re defending any individual conviction that we’re defending Scarcella,” said Eric Gonzalez, the acting Brooklyn district attorney. “We just never found any allegations of specific misconduct — there’s not been a smoking gun.”

Intentions notwithstanding, by stating on the record that it has no evidence that Mr. Scarcella committed any crimes, the district attorney’s office has relieved itself of a handful of unpleasant consequences. If prosecutors had such evidence and made it public, defense lawyers would have powerful new leverage in their efforts to free defendants who contend that Mr. Scarcella helped put them into prison wrongly.

Image Ken Thompson, then the Brooklyn district attorney, said of Mr. Scarcella in one case, “We can’t say he acted inappropriately.” Credit... Robert Stolarik for The New York Times

A finding of misconduct could also make civil claims of liability considerably easier to win. The city and the state have so far paid more than $40 million in claims related to Mr. Scarcella, and other cases are pending.

Ronald L. Kuby, a lawyer representing a number of defendants who claim that the former detective mishandled their cases, said the prosecutors’ stance toward Mr. Scarcella had discouraged a deeper examination of the criminal justice system. By arguing that Mr. Scarcella did nothing wrong, Mr. Kuby said, the district attorney’s office has made it easier to say the same about the prosecutors who presented his cases to juries — and about the judges who heard those cases and sentenced people to prison. Many of those judges and prosecutors still work in Brooklyn today.