In 1868, Madame Guyot was found murdered in Montana, where she operated a hostel for travelers along what is now known as MacDonald Pass. Even after 155 years, the secret of who killed her is still intact. 

 

Sources:

https://www.distinctlymontana.com/unsolved-montana-murders

https://missoulacurrent.com/harmon-old-frenchwoman/

https://www.hmdb.org/m.asp?m=127754