When teenager Gary Hall was gunned down on a Nicetown street in 2006, his friends all seemed to know who did it.

They blamed Ivan Simmons, whose little brother had been seen arguing with Hall minutes earlier. A few months later, Simmons was fatally shot in what Hall’s friends told police was an act of revenge for Hall’s murder. And after that, a friend of Simmons shot Hall’s cousin — retaliation for the retaliation.

Yet Philadelphia police disregarded those statements, court filings show. Instead, they arrested someone else: David Sparks, a 16-year-old who’d been picked up for violating curfew the night of Hall’s killing.

Sparks was convicted and served 15 years in prison for Hall’s murder. He was exonerated Monday after the District Attorney’s Conviction Integrity Unit (CIU) and the Pennsylvania Innocence Project unearthed extensive, previously undisclosed evidence in police files pointing to Simmons as the shooter.