Theophalis Wilson was released on Tuesday after a Philadelphia court vacated his wrongful conviction and dismissed all charges against him. He is the 12th person exonerated by District Attorney Larry Krasner’s Conviction Integrity Unit (CIU).

In 1992, Mr. Wilson was arrested for killing three people in North Philadelphia. The crime occurred in 1989, when Mr. Wilson was just 17 years old. He and his 29-year-old co-defendant, Christopher Williams, were convicted in 1993. Mr. Wilson was sentenced to life imprisonment without parole and Mr. Williams was sentenced to death.

As the Philadelphia Inquirer reported, the prosecution’s case turned on the testimony of James White, a six-time convicted first-degree murderer who claimed to be an accomplice in the murders. Prosecutors admitted in a court filing in 2001 that they had no case against Mr. Wilson or Mr. Williams “unless the jury believed White.”

At a 2013 hearing in Mr. Williams’s case, James White admitted that he lied at trial in exchange for a plea agreement to avoid the death penalty. Forensics experts also testified that the physical evidence discredited Mr. White’s story that the three men were shot and pushed out of a moving van. The court granted Mr. Williams a new trial because his trial lawyer failed to present this expert evidence, which “would likely have changed the jury’s mind” about believing Mr. White and resulted in acquittal.

But even after its crucial witness was completely discredited, the District Attorney’s Office planned to retry Mr. Williams. In response to pretrial discovery requests, prosecutors steadfastly refused to turn over their case file to the defense.

More than half of wrongful convictions can be traced to witnesses who lied in court or made false accusations. False testimony by jailhouse informants is especially common in death penalty cases.

“Everything that goes wrong in the criminal system goes wrong worse in capital cases,” Robert Dunham, a former federal defender in Pennsylvania who heads the Death Penalty Information Center, told the Inquirer. “Generally speaking, the more high-profile a case is, the more prone it is to government misconduct. There is greater public pressure and greater political pressure to solve the case and to convict somebody. There is greater political gain to a prosecutor who has ambitions beyond his office.”

Meanwhile, Mr. Wilson spent years trying to challenge his conviction without legal help. He managed to get his case into court only after the U.S. Supreme Court struck down mandatory life-without-parole sentences for children in Miller v. Alabama, entitling Mr. Wilson to a resentencing hearing—and a lawyer. Mr. Wilson persuaded his resentencing lawyer to help file his innocence claim, which prompted the CIU to open a review of both his case and Mr. Williams’s in May 2018.

Last March, the CIU gave defense lawyers over 40,000 pages of documents from the prosecutor’s file that had never been disclosed. Lawyers told the Inquirer the files revealed “a catalog of deep problems in Pennsylvania’s legal system, [including] concealed evidence, undisclosed deals in exchange for testimony, corrupt relationships with informants, and a direly inadequate system of appointing and funding defense counsel that doomed both men to decades in prison.”

CIU chief Patricia Cummings asked the court to drop the charges against Mr. Williams because evidence in the file confirmed that Mr. White’s testimony was false and there was no other evidence against Mr. Williams or Mr. Wilson. The court granted that request in December.

Citing Philadelphia prosecutors’ violation of Brady v. Maryland, the U.S. Supreme Court decision requiring prosecutors to disclose evidence that’s helpful to the defense, the CIU also moved to withdraw all charges against Mr. Wilson.

“For decades and with some frequency, it appears that the Philadelphia District Attorney’s Office failed to comply with its obligations in regard to Brady,” Ms. Cummings wrote in a filing in Mr. Wilson’s case, which she described as a “perfect storm” of injustice. “Wilson’s trial was infected by serious prosecutorial misconduct, Brady violations, a critical witness who supplied false testimony, and ineffective assistance of counsel.”

“It is time for Mr. Wilson to be allowed to go home — that he go home a free man, and that he go home with an apology,” Ms. Cummings argued in court on Tuesday. “No words can express what we put these people through. What we put Mr. Wilson through. What we put his family through.”

The trial court found that Mr. Wilson’s right to due process, effective counsel, and any material exculpatory evidence in his case had been violated and ordered that he be released immediately.

“This is a great day,” Mr. Wilson, now 48, said outside the courthouse. “Now we got to go back and get the other guys. There’s a lot of innocent people in jail.”