On the fifth day of the trial of a former narcotics detective accused of inventing evidence, a prosecutor made a startling admission.

The prosecutor, Stephanie Minogue, said that she did not turn over reports related to the case, in part because she had originally thought that the material was “irrelevant.”

But the judge noted that it was not the first time that Ms. Minogue had failed to hand over potential evidence about the detective, Joseph Franco, who was charged with perjury and official misconduct in 2019 and whose involvement led to the collapse of hundreds of cases around New York City.

“This is extraordinarily late, and worse than that, much, much worse than that, it is now a pattern,” said the judge, Robert M. Mandelbaum. He called the delayed handoff of the material to Mr. Franco’s lawyer “outrageous.”

That exchange, memorialized in court records obtained by The New York Times, marked the beginning of the end of a case that had become a symbol of officer misconduct at a time when scrutiny of the police had intensified around the country. Mr. Franco was fired from the Police Department in 2020.

Now he will go free.

Justice Mandelbaum on Tuesday dismissed the case outright after prosecutors — noting that Ms. Minogue was no longer on the trial team — admitted that yet another portion of potential evidence, known as discovery, had not been fully reviewed.

“This office can no longer represent to the court that we have provided all the material that should be discovered, even following those reviews,” said Nick Viorst, Ms. Minogue’s boss and the head of the district attorney’s Police Accountability Unit.

Given the statute of limitations, Mr. Franco is unlikely to be charged with similar crimes in other boroughs, and the accountability unit is under scrutiny after bungling the high-profile case.

When the Manhattan district attorney, Alvin L. Bragg, was running for office, he vowed to create a Police Integrity Unit to replace it. Instead, he maintained the unit he had inherited from his predecessor, which had been overhauled after the murder of George Floyd in May 2020, and kept Mr. Viorst as its supervisor. One major change did occur: The unit now reports directly to Mr. Bragg.

“D.A. Bragg has prosecuted public corruption and police misconduct throughout his career, and has made this work a priority as district attorney,” said Emily Tuttle, a spokeswoman for the office. “When discovery issues dating back to 2019 arose in this trial, D.A. Bragg was immediately informed and deeply involved in correcting these errors where possible, and determining next steps.”

She said that Mr. Bragg usually met with the unit once every two weeks and has met with them about two dozen times since taking office.

Mr. Viorst’s appointment as head of the unit was announced in January 2021, with Ms. Minogue joining it the following month. In April, she became its deputy, a position she held until Tuesday, when the office removed her from her post.

Mr. Viorst had also worked for Mr. Bragg when both men were at the New York attorney general’s office, leading a unit charged with investigating police killings of unarmed civilians. It is a difficult job — the law makes it notoriously hard to even charge police officers, let alone win a guilty verdict — but under their supervision, the unit brought back zero convictions in the 24 cases it investigated. (It has had no luck since, either; the unit, now called the Office of Special Investigation, still has not won a conviction.)

It is unclear whether Mr. Bragg and Mr. Viorst could have salvaged the case against Mr. Franco after the evidence violations. Had they not occurred, there are indications that there would have been a strong case against the ex-detective.

One juror, Daniel Keehn, 30, said prosecutors had presented persuasive arguments against Mr. Franco.

“The prosecutors were doing a thorough job of laying out the case,” he said. “They did a good job building the foundation, going over the evidence.”

Ms. Minogue questioned several of Mr. Franco’s former colleagues and supervisors, and many were reluctant to condemn the former detective’s work. None said they believed he had misinformed them, even after they reviewed the prosecution’s video evidence.

The prosecution’s strongest witness was Timothy Heil, an assistant district attorney who expressed doubts about Mr. Franco’s claims that he saw hand-to-hand drug deals, Mr. Keehn said.

“Heil comes on and he’s breaking down the video evidence,” Mr. Keehn said. “He’s really piecing together a lot of the picture.”

The Bronx and Brooklyn district attorneys together abandoned more than 430 convictions that were based in part on Mr. Franco’s work. Their representatives noted that he had been fired from the police force after a department trial found he testified falsely before a grand jury. Manhattan prosecutors, who have dismissed more than 100 cases in which he was involved, say they are still scrutinizing others.

When Mr. Franco was charged three years ago, prosecutors said that they had discovered that his accounts of witnessing separate drug transactions involving three people, all of whom pleaded guilty, had been fabricated. Prosecutors in Manhattan, as well as the Bronx and Brooklyn, where Mr. Franco had been stationed previously, began to dig into his 20 years of detective work, during which he made thousands of arrests.

“This detective lied to judges, prosecutors, and his own colleagues in the N.Y.P.D. about crimes that never happened, and three New Yorkers wrongfully lost their liberty as a result,” Mr. Vance said in a statement at the time.