Dennis Lynn Rader is an American serial killer known as BTK, the BTK Strangler or the BTK Killer. Between 1974 and 1991, BTK killed ten people in Wichita and Park City, Kansas. Rader pleaded guilty and was sentenced to to 10 consecutive life sentences.

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.