James Washington from Nashville, thinking he was going to die as he was suffering a heart attack, was clearing his conscience.
He confessed to killing Joyce Goodener in July 1995, reports WSMV.
However, Washington - who was already in prison for a different crime - survived the scare and has since been found guilty of the murder.
Prison guard James Tomlinson recalled to the jury: "He kind of got as best as he could, motioned and said, 'I have something to tell you. I have to get something off my conscience and you need to hear this'.
"He said, 'I killed somebody. I beat her to death'."
Washington had tried to retract his statement in court before the verdict was delivered.
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