Dennis Lynn Rader Wichita Kansas' Bind Torture Killer serial murderer aka BTK.  

One page letter from one of Rader's pen pals written to another friend of Rader's in which they are thanking the recipient for forwarding something from Rader. 
Includes the original mailing envelope postmarked April 7, 2017.

Dennis Rader Pen Pals Letter and Envelope Set