in the neighboring town of sinton had just been given a letter addressed to kristene chapa's father. does the letter say? >> the letter is written from the perspective of a hit man who has been hired to kill the surviving victim in this case, kristene chapa. >> in the letter, the hit man even named the person who had hired him. if you're wondering, the letter does not mention dylan spellman. all of it was a twist as big as texas. texas. >>> coming up -- the mysterious letter. could this be the smoking gun police had been looking for? >> immediately, i knew this was something very big and very important. >> when "dateline" continues. >> when "dateline" continues >>> while detective aaron volaman continued trying to strengthen his case against dylan spellman, investigators in neighboring sinton, texas, received a mysterious letter addressed to kristene chapa's dad. >> immediately i knew this was something very, very big and very important. >> the letter-writer described himself as a hit man hired by the person who shot mollie and kristene. he wrote, the gunman had sought his services t