Illustration by Anna Parini

A couple of times a year, Peter Forcelli, a former New York Police Department detective who now works as a federal investigator in Florida, returns to the city to testify in court cases. The night before his appearances, he often heads to El Rio Grande, a midtown Tex-Mex restaurant, where he and former N.Y.P.D. colleagues trade stories about their days on the force. Forcelli, who is fifty-one, worked in the Bronx during some of the most violent years of the nineteen-nineties, before he left for a job with the Bureau of Alcohol, Tobacco, Firearms, and Explosives. As a federal agent, Forcelli has found himself developing an unusual specialty: reinvestigating cases that were considered solved and uncovering evidence to free people imprisoned for crimes they didn’t commit. His view of this work is uncompromising, even a little heroic, colored by the cop dramas he watched growing up, and his determination to expose flawed tactics and shoddy work has aroused the anger of officers and prosecutors.

Last year, a hundred and fifty-seven people nationwide were exonerated—a record number. Amid growing awareness of the scope of the problem, the National Registry of Exonerations has gathered data on wrongful convictions going back to 1989, logging nearly two thousand exonerations, and the number is only a fraction of the convictions that are now being contested on the ground of innocence. In the popular imagination, blame for wrongful convictions falls on individuals: the racist prosecutor, the crooked cop. Although such cases do occur, Forcelli has come to believe that the problem is more fundamental—that the workings of the criminal-justice system itself have led even conscientious prosecutors, judges, and juries to put innocent people in prison.

On a visit to the city in October, 2014, Forcelli went to the Bronx Supreme Court to testify in a hearing relating to a case from 1995, in which a young man named Edward Garry had been convicted of killing a retired N.Y.P.D. detective in a holdup at a grocery. The conviction was based on testimony from two eyewitnesses, but Garry had always maintained his innocence, and the hearing was part of his fifth formal plea for exculpation. As Forcelli sat in the witness box, he was increasingly uneasy. Garry’s case seemed to be one of those in which everyone did what they were supposed to do, but something had gone wrong, costing an apparently innocent man twenty years of his life. One thing made the case especially unsettling: Forcelli himself had helped arrest Garry. “When I sat in the chair and I looked at Garry, I had this horrible, sinking feeling that I knew exactly what happened,” Forcelli told me. “That kick-in-the-chest feeling I had hasn’t gone away since.”

Forcelli grew up in Bedford Park, in the Bronx. His was one of the few white families on the block. His father was a construction worker, and jobs were sporadic; sometimes the family ate Cheerios for dinner. Money problems drove the family to move in with his dad’s parents, in South Yonkers—fourteen family members from three generations, all crammed into one house. Forcelli always knew that he wanted to become a policeman. He took the entrance test for the N.Y.P.D. at sixteen, four years before he was eligible to join the force, and finished high school and worked as a carpenter until he could apply.

In 1986, at the age of twenty, he became a housing officer, and was assigned to patrol projects in the East Bronx. It was during the crack-cocaine epidemic, and that year there were more than sixteen hundred murders in the city, almost five times the number today. But Forcelli took to the work immediately. When he saw children heading to school or celebrating birthdays, they reminded him of the kids he grew up with, and he was shocked by the attitude of some of his colleagues. “You see this ‘Ah, people in the projects, they’re all animals,’ ” he said. He soon gained a reputation for investigating crimes until there was nothing left to investigate, and his superiors found him driven and outspoken. “He wasn’t afraid to let you know if he thought there was a better way to do things,” one of them recalled.

Forcelli has always been obsessive about his work. He doesn’t follow sports, or music, or politics, and is dreading mandatory retirement, when he turns fifty-seven. When I visited him at his A.T.F. office, in Miami, I noticed that the pens and papers on his desk were arranged with a precision bordering on the fanatical. He wears a dark suit to work almost every day. (Occasionally, he permits himself a palm-covered Tommy Bahama shirt, a taste he attributes to watching too much “Magnum, P.I.” as a kid.) He refers to crime victims respectfully, using “Mr.” or “Mrs.,” and can precisely recall crime-scene addresses from decades ago. Early in his career, his office was next to an interrogation room, on one side of a two-way mirror; as he ate his lunch, he would study the techniques that seasoned detectives used in questioning suspects. After he married, in his late twenties, his wife, Noreen, whom he’d met at a police fund-raiser, would bring him clothes and food as he worked double and triple shifts.

In 1995, Forcelli was promoted to detective and assigned to the N.Y.P.D.’s Forty-seventh Precinct, which stretches from Woodlawn Cemetery, in the western part of the Bronx, to the New York State Thruway, in the east. Its main roads are flanked by malnourished trees and low-rise buildings with faded awnings—storefront churches, transmission-repair shops, dollar stores. The year that Forcelli began working there, the precinct was the city’s most dangerous, with more murders than anywhere else in the five boroughs. Almost every day, officers were confronted with knifings, shootings, disembowelments, and “dump jobs”—bodies that showed up in parks or rivers. Three weeks into the job, he got his first fresh case as a detective, the murder of a retired cop named Oswald Potter.

The murder took place at Irene’s New Hope Grocery, which occupied a small corner building on Laconia Avenue. The front room of Irene’s was an ordinary grocery, but tucked in a parlor behind it was an illegal gambling den. Men gathered nightly to drink beer and play dominoes or the numbers game (a kind of unofficial lottery). In the early evening of August 18th, four regulars were playing dominoes around a table. Gladys Garcia, a clerk who oversaw the gambling operation, and Oswald Potter were looking on. Potter, who was sixty-two, had retired as an N.Y.P.D. detective in 1982, after twenty-five years working in the Bronx.

At 6:20 P.M., a man burst through the door to the back room. “This is a stickup!” he shouted, waving a gun. “Everybody down!” Another armed man followed him. Several things happened almost simultaneously: the men playing dominoes dived under the table; the first man handed Garcia a plastic bag and told her to fill it with cash; and Potter yelled, “I’m a police officer!” Potter knocked the second man to the floor, and made a grab for his gun. Garcia saw him run into the front of the grocery, pursued by the first man, and then lost sight of them. Soon afterward, she recalled, “I heard a lot of shots.”