Johnny Cash held his fair share of prison concerts over the years, but the album At Folsom Prison, recorded at California's infamous maximum security prison, is arguably his most famous. Though it wasn’t his first time performing for inmates, the Folsom Prison show had extra meaning, thanks to Cash's 1955 song “Folsom Prison Blues.”

But on January 13, 1968, it wasn’t “Folsom Prison Blues” that stole the show. The night before, a chaplain had slipped a tape to the Man in Black, explaining that there was an inmate who passed the time by writing songs, and that his stuff was pretty good. Cash took a listen, then spent the rest of the evening learning the tune with his band.

The next day, after playing his whole set for the incarcerated men, Cash announced that his final song, “Greystone Chapel,” was something extra special. “This next song was written by a man right here in Folsom Prison, and last night was the first time I’ve ever sung this song,” Cash explained. “Anyway, this song was written by our friend Glen Sherley ... Hope we do your song justice, Glen. We gonna do our best.”

It was just the beginning of what Cash would do for Glen Sherley, who was serving time for armed robbery. Word of Sherley’s songwriting ability got out, and in 1971, country music legend and Grand Ole Opry member Eddy Arnold recorded a Sherley tune called “Portrait of My Woman.” This was enough for Sherley to secure a record deal, which he recorded in prison.

In the meantime, Cash was working behind the scenes to help his convict pal, and managed to convince then-Governor of California Ronald Reagan to grant Sherley early release. When he got out on March 7, 1971, Johnny and June Carter Cash were waiting at the gate with a publishing contract. Sherley hopped on a plane to Nashville, and made his stage debut just one month later on tour with Cash.

Sherley even found love thanks to his House of Cash connections, marrying a secretary who worked in Cash’s Hendersonville, Tennessee business office. But years in prison had affected the man, and his rough edges began to show through. After Sherley caused some scheduling problems due to staying up too late and sleeping in too long, bassist Marshall Grant was elected to discuss the matter with him. “He seemed to be taking it very well,” Grant later wrote, “But then he sort of opened up and gave me a chilling insight into his personality.”

Sherley’s response was a far cry from how one would normally respond to a critique from your boss: “I’d like to take a knife and start right now and just cut you all to hell. It’s not because I don’t love you, because I do. But that’s just the type of person I am. I’d rather kill you than talk to you.”

It wasn’t the first time Sherley had said or done something disconcerting. Concerned about his continued violent tendencies, Cash decided to remove Sherley from the tour—and from the House of Cash entirely. Though he tried for several years to make a go at a “normal” life, Sherley had trouble adjusting. He died of a self-inflicted gunshot wound in 1978. Cash paid for the funeral.