http://unsolvedmysteries.wikia.com/wiki/Jay_Cook_and_Tanya_Van_Cuylenborg
Jay Cook and Tanya Van Cuylenborg went missing after what was supposed to be a day trip to Seattle. After not arriving, and not calling, Tanya was found partially clothed but raped and strangled. Police began to suspect that Jay killed Tanya... until Jay's lifeless body was found bound under a bridge. What made the case even CRAZIER was that sometime after the murders took place, the families of the victims started receiving cards in the mail from all over the US being taunted with the deaths of the two teens.
According to this, the police originally believed the letters to have been written by someone with some connection to the crime (and it was determined they were all written by the same person). However, in 2010 they identified the writer as a man in his 70's with "severe mental issues" and after questioning him, they don't believe he had any involvement with the murders.