Kenneth Thompson, in 2012. Robert Stolarik / The New York Times / Redux

Last Tuesday, toward the end of an hour-long meeting, Kenneth Thompson, the district attorney of Kings County, New York, exhaled heavily several times, as though by breathing he could expel the dilemma in front of him. The directors of a conviction-review unit he formed early last year had just explained their recommendation that he move to vacate Derrick Hamilton’s 1993 second-degree-murder conviction, for which Hamilton had served nearly twenty-one years in prison before being let out on parole. Thompson had had about two weeks before the meeting to review the case and a corroborating recommendation from an independent review panel, and now he had to decide whether the evidence exonerated Hamilton.

Just over twenty-four years ago, on January 4, 1991, a man named Nathaniel Cash was shot and killed outside a brownstone in Bedford-Stuyvesant, Brooklyn. In March of that year, Hamilton was arrested for the murder. He was twenty-seven at the time, and had previously been convicted of manslaughter, but was out on parole. Detectives on the case relied largely on one eyewitness, Jewel Smith, Cash’s girlfriend, to build their case. Although she had told the first detective on the scene that she hadn’t seen the shooting, she ultimately identified Hamilton as the murderer; at trial, he was found guilty, and he received a sentence of twenty-five years to life in prison.

Defense attorneys had focussed their case on alibi witnesses who contended that Hamilton was in New Haven, Connecticut, on the day of the murder. Following Hamilton’s conviction, in 1993, he and his lawyers maintained this narrative—to no effect, even after Smith recanted her testimony. In 2011, Hamilton was released on parole, with his conviction still in place, and in January, 2014, prompted by Hamilton’s lawyers, Thompson’s office began reviewing the case.

One day last year, Mark Hale, the assistant district attorney who runs the Kings County conviction-review unit, visited the scene of the Cash murder. In Smith’s account, which prosecutors had relied on to convict Hamilton, she had said that Hamilton shot Cash in the chest while they were standing in the entrance of the brownstone. She claimed that Cash then walked out of the house and up some stairs, then collapsed on the curb of the sidewalk, where he died.

Hale already had reason to doubt the account. There was Smith’s recantation, for one, and, what’s more, forensic evidence had contradicted her assertion that Cash had been shot in the chest while standing in the building. When Hale saw the brownstone, his suspicions increased. The vestibule was only about six feet wide and five and a half feet deep, a setting inconsistent with ballistics evidence and the medical examiner’s report. Neither Cash’s body nor his clothes had shown evidence of a close-range shooting, and the location of the discharged shell casings indicated that the shooter had stood in the vestibule and shot outward, in the direction of the street. Hale also saw that Cash would have had to walk down a set of stairs, contradicting Smith’s assertion.* Moreover, Cash’s autopsy found evidence of injuries that would have made it impossible for him to walk after the fatal shot.

These problems proved decisive for Thompson. He decided to submit his motion to a judge, and, if the judge assents, Hamilton will become the eleventh person Thompson’s office has exonerated since he took office in January, 2014.

The Conviction Review Unit has been the most profound reform that Thompson has implemented in his year as district attorney. A team of ten lawyers has been tasked with reviewing wrongful-conviction claims and questionable convictions, many of which occurred under the leadership of the previous D.A., Charles Hynes, whose twenty-three-year tenure is suspected of being marked by negligence and questionable ethics—including using faulty eyewitnesses, manipulating his prosecutorial responsibilities in order to appear tough on crime and win elections, and relying on the work of discredited detectives. One retired detective in particular, Louis Scarcella, has been connected with roughly seventy cases that have come up for review by Thompson’s office, including Hamilton’s. Meanwhile, one of Hynes’s assistant district attorneys, Michael Vecchione, was named in a wrongful-conviction lawsuit brought against the city by Jabbar Collins, who spent sixteen years in prison for murder. Collins claimed that Vecchione and others in the prosecutor’s office had threatened a man in order to solicit testimony of Collins’s guilt. (Collins was awarded a settlement of ten million dollars last summer. Both Scarcella and Vecchione deny any wrongdoing.)

The C.R.U. represents Thompson’s attempt to correct systemic flaws in Brooklyn’s criminal-justice apparatus, which have included poor oversight, inadequate independent review, and a lack of prosecutorial and police transparency—and which have enabled problems ranging from mistakes in judgment to deliberate misconduct. Thompson’s is the third-largest district attorney’s office in the nation, behind those of Chicago and Los Angeles, with five hundred prosecutors who litigate roughly a hundred thousand cases a year, and it is certainly the largest to make such a thorough effort to review past convictions. In scope, the Kings Country C.R.U. follows an earlier effort by Craig Watkins, the district attorney in Dallas, who, in 2006, formed a conviction-integrity unit that sought, at first, to review potentially tainted convictions that could be tested with DNA evidence that wasn’t available at the time of the original trials.

Thompson’s unit differed from Watkins’s in that it sought to consider an expanded notion of justice. “They’re not simply looking at wrongful convictions in cases in which a person can prove his or her innocence. They’re also looking at cases where they may be innocent—we don’t know—but, definitely, the conviction has no integrity,” Peter Neufeld, the cofounder of the Innocence Project, told me. Watkins later expanded his unit in Dallas to include convictions not resting on DNA evidence, but Thompson’s office has not yet widened its scope to include cases in which retroactive DNA testing can be applied. Rather, the questionable convictions that the Kings County office has sought to review can largely be traced to human error—negligence, misconduct, or errors in judgment—and not necessarily to poor technology. “It is much more difficult to set aside convictions in non-DNA cases, so Kenneth Thompson’s work in that regard has been especially impressive,” Karen Daniel, the co-director of the Center on Wrongful Convictions at the Northwestern University School of Law, wrote to me in an e-mail.

Thompson talks about justice with a more philosophical tone than one might expect from an elected official. “I believe that too many D.A.s view their duty as to convict,” he told me. “I think that the main duty of the D.A. is to do justice. That means to protect public safety, but it also means that we have to ensure that our criminal-justice system is based on fundamental fairness.” Thompson, who is forty-eight, spent his early childhood in Harlem’s Wagner Houses. In 1973, his mother became a New York City police officer, which allowed her to move her three children to the Co-Op City neighborhood of the Bronx. “My mother was part of the first group of women—black or white—to go on patrol in the history of the city,” Thompson told me. “To think that she could become a police officer in 1973, when there were no examples for her, was really extraordinary. She left an indelible impression on me about law enforcement, about issues of fairness and justice.”

Among the artwork hanging on Thompson’s office walls is a framed courtroom sketch of his opening statement in the 1997 trial of Justin Volpe, an N.Y.P.D. officer accused of beating and sodomizing a Haitian immigrant named Abner Louima. Thompson was an assistant U.S. attorney at the time, part of a team of federal prosecutors who litigated the case and ultimately secured a guilty verdict against Volpe. His involvement in the Louima case speaks to his awareness that the system is not always right, and that justice ought to be applied to everyone—an awareness that has carried into his aims in forming the C.R.U. “Wrongful convictions, for a lot of reasons, destroy the lives of the people who have been wrongfully convicted, and their family members,” Thompson told me. “But they also do damage to the integrity of the system.”