真犯人

しんはんにん【真犯人】

the real [actual, true] criminal [culprit, perpetrator]; the person who really did it [committed the crime].

●時効寸前に真犯人が名乗り出た. Just before the statute of limitations ran out, the real criminal [actual culprit] came forward [confessed].

●真犯人を突き止める find the real criminal [culprit, perpetrator]; hunt down the person who really did it [committed the crime].