When he was in high school, he and some buddies robbed a house. One of the things they stole was a gun. He then used fake id, bought bullets for the gun, and loaded it. His prints were left on the gun and bullets. One of the guys used the gun to murder someone a few days later. Cops tracked down Johnson.
He made a good witness for the prosecution’s case against the actual shooter so prosecution arranged to have him admit his involvement and rat the other guy out and in exchange he avoided jailtime.