tonight, a story of doggedness and perseverance, a hunt for the truth and the meaning of justice that have carried an imprisoned man and an nbc news journalist on an unlikely two-decade-long journey in search of answers. the sing sing correctional facility houses more than 1,400 prisoners. each can tell you why they re here. dear mr. slepian, i spent a lot of time some will tell you why they shouldn t be. i know i don t belong here but i am a firm believer that everything that happens to us in life is for a purpose. reporter: it was a sinal letter addressed to dateline nbc producer dan slepian