For nearly 20 years, one photograph stood between Sheldon Thomas and freedom.

It was a picture of the wrong Sheldon Thomas.

In 2004, police officers showed the image of a young Black man to a witness, who chose him from an array of six as a suspect in a fatal shooting in Brooklyn’s East Flatbush neighborhood. That identification withstood scrutiny through an indictment, trial and appeals over more than 18 years.

But now, the Brooklyn district attorney’s office is saying that detectives, prosecutors and the original trial’s judge knew from the outset that the photo in the array wasn’t actually of the man they wanted to arrest, but they proceeded anyway.

In a new report from the Brooklyn district attorney’s conviction review unit provided to The Times, prosecutors said that the two men shared a name, and they had addresses in the same precinct, but police investigators knew early on that they were different people.

Mr. Thomas, 35, is scheduled to appear in court on Thursday afternoon before Matthew J. D’Emi, a judge with the Brooklyn Supreme Court. The district attorney’s office said in its report that the conviction should be vacated.

The case was “compromised from the very start by grave errors and lack of probable cause to arrest Mr. Thomas,” District Attorney Eric Gonzalez said in a statement.

“He was further deprived of his due process rights when the prosecution proceeded even after the erroneous identification came to light,” Mr. Gonzalez said, calling the conviction “fundamentally unfair.”

Attorneys for Mr. Thomas declined to comment before the hearing.

The case would be the 34th conviction vacated after re-investigations by the unit, which was expanded in 2014, and shows what can happen when checks in the criminal justice system break down.

The prosecutor’s office said that the man in the photo array and Mr. Thomas do not look alike, despite the assertions by police investigators, government lawyers, the trial judge and an appellate panel.

In a defense-commissioned study, prosecutors said, 32 law students of color examined a photo of Mr. Thomas, who is Black. Then they looked at the photo array. Twenty-seven correctly said Mr. Thomas was not in it.

The case that was the undoing of Mr. Thomas’s life started on Christmas Eve 2004: People in a car shot at six others on the corner of East 52nd Street and Snyder Avenue in Brooklyn, killing a 14-year-old boy, Anderson Bercy, and wounding a man.

Detectives had “repeatedly harassed” Mr. Thomas for months following a previous arrest for pointing an inoperable gun at police officers, the district attorney said.

After the killing, investigators zeroed in on him, obtaining the photo of the man with the same name, according to the report. They then prompted a witness to choose that incorrect picture, and arrested the Mr. Thomas they wanted, the report said.

Mr. Thomas denied being in Brooklyn on the night of the shooting, telling investigators that he was in Queens until 3 a.m. on Dec. 25, according to the report.

But then, the report said, Mr. Thomas was wrongly identified during three in-person lineups; prosecutors most likely failed to disclose false police testimony; they used a witness with questionable credibility; and a defense counsel exacerbated the errors.

Investigators had coerced the witness, guiding her, the report said. Her photo identifications “were prompted by the detectives,” the report says. “The detectives were intent on arresting defendant.”

During a pretrial hearing a year and a half after Mr. Thomas was arrested, the lead detective admitted on cross-examination that he had provided false testimony about the photo array, prosecutors said.

Later, another detective said that Mr. Thomas was also on the police’s radar because of an anonymous tip, but acknowledged that Mr. Thomas did tell them he was not the man in the photo.

However, the judge determined that the photo of the wrong man was of no consequence, that the men resembled one another, and that the police had other evidence giving them probable cause to make the arrest. The trial went on.

While a co-defendant was acquitted, Mr. Thomas was convicted of second-degree murder, attempted murder and other counts and was sentenced to 25 years to life in prison.

The repeated mistakes and misdirection meant that Mr. Thomas was unjustly deprived of his freedom, prosecutors wrote in the new report.

“Each of these errors, on its own, deprived defendant of a fair trial,” they wrote. “Together the errors undermined the integrity of the entire judicial process and defendant’s resulting conviction.”