“I think there was a lot of pressure to solve these homicides,” said Mr. Figorski, who retired in 2006 as a lieutenant in the department’s narcotics unit.

Mr. Thomas had been sentenced to life in prison without the possibility of parole. One of the co-defendants was offered a plea deal and was released from prison in 1999, Mr. Figorski said. Mr. Thomas was also offered a deal — to serve five to 10 years — but he maintained his innocence and turned down the offer.

His appeals meandered for years in state and federal courts as he tried on his own to overturn his conviction.

A letter from Mr. Thomas was waiting for the Pennsylvania Innocence Project when it opened its doors in 2009, Marissa Bluestine, its legal director, said on Thursday.

For eight years, Ms. Bluestine and Mr. Figorski sought various remedies in court. Last year, they met with members of the Conviction Review Unit from the Philadelphia district attorney’s office. Investigators interviewed one of the co-defendants, who told them that he was not involved or present at the murder and that he lied at Mr. Thomas’s trial, according to a timeline assembled by Mr. Thomas’s lawyers.

About two weeks ago, an investigative file that had long been sought by defense lawyers was found in a box in a hallway at police headquarters. The file included witness statements that contradicted accounts that Mr. Thomas was involved.

“Had that information been available at trial — and had the story of Shaurn’s presence in court at the moment the murder was committed been told correctly — prosecutors agreed the trial would likely have ended differently,” the Pennsylvania Innocence Project said in a statement.