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In February 2012, Richard Miles, wearing his late father's suit, walked out of a packed Dallas courtroom and into the arms of his mother a free man. He'd spent 15 years in a cold, tiny cell. "In that moment, life was breathed back into me," says Miles, 37, who now runs an inmate support group. "Finally, someone was able to prove me innocent." That someone was Jim McCloskey, a businessman turned lay minister, whose 15-month investigation cracked Miles's case wide open. The smoking gun? McCloskey and his band of investigators at the Princeton, N.J.-based Centurion Ministries found a lost police memo which revealed another man had confessed to the 1994 killing. "It was buried treasure," says McCloskey. "There's nothing more beautiful than the truth."

That insatiable belief that the truth will set the innocent free has been a driving force for McCloskey, 70, a self-taught sleuth whose team recently helped release its 50th inmate since his first case in 1980. Run solely on donations (investigations can cost a half million dollars), Centurion's staff of seven, none of whom have police training, can spend years re-interviewing witnesses, poring over documents and tracking down sources police never talked to. "Centurion is a ray of hope for the innocent languishing behind bars," says Steven Drizin, a Northwestern University law professor. "They're a lifeline."

For Clarence Brandley, a custodian accused of raping and killing a Texas high school student, McCloskey was a life-saver: He helped stop the 1987 execution of Brandley just eight days before his lethal injection. "He's an angel," says Brandley, who walked off death row three years later with McCloskey behind him. "The guards and inmates were cheering," he recalls. "They shut the prison down for me."

Raised in suburban Philadelphia, McCloskey, a lifelong bachelor, left a lucrative corporate executive job at age 37 for Princeton Theological Seminary in search of purpose. During his second year he served as a student chaplain at the New Jersey State Prison, where he met Jorge De Los Santos, a heroin addict accused of murder. "He challenged me to free him," says McCloskey, who succeeded three years later. "Jorge gave me a mission in life."

Although the inmates they've helped free have collectively served 972 years, most have not been compensated, and only one real killer has been brought to justice. "There's a lot of heartache in this work," says McCloskey, "but to have a mother thank us for bringing her son or daughter home, that's all the reward we need."