“This case involved a perfect storm of error — bad defense counsel, an unreliable witness, critical evidence that was never disclosed to the defense,” Ms. Saifee said.

When the killing occurred, on Feb. 18, 1991, Mr. Hatchett was 24 and hobbling on crutches after being hit with bullets in the throat and leg as a bystander in a shooting the previous year. He had an I.Q. of 63, with the reading and writing ability of a first grader, according to lawyers from the Innocence Project.

Mr. Hatchett and Ms. Carter had seen each other earlier on the evening of the murder, in a rooming house where his aunt lived alongside Ms. Carter and her mother. Mr. Hatchett gave Ms. Carter money to buy crack, his lawyers said in an interview. She left around 9:30 p.m. and never returned.

Though he cooperated with the police and provided an alibi, Mr. Hatchett was arrested and convicted almost entirely on the testimony of a career criminal named Gerard Williams, who said that he had seen, from 30 to 40 feet away, Mr. Hatchett striking a body on the ground in the park that night.

Mr. Williams offered the account after he was arrested in connection with a burglary a little over a week after the killing, and after having initially identified someone else as the killer — information the prosecutors never gave the defense, as was required.

The defense itself was so incompetent that a judge declared Mr. Hatchett’s first trial a mistrial. But even at the second trial, Mr. Hatchett’s lawyer failed to present evidence of his intellectual disability or of the fact that his injuries would almost certainly have prevented him from striking Ms. Carter with the force Mr. Williams had described or lugging her body across the park while leaning on crutches.

“It’s frightening how easy it is to convict an innocent person in this country,” Ms. Saifee said. “And it’s overwhelmingly difficult to release an innocent person.”