The "Lonely Hearts Killers" were American.
Lonely Hearts is a 2006 American film directed and written by Todd Robinson. 
 It is based on the true story of the notorious "Lonely Hearts Killers" of the 1940s, Martha Beck and Raymond Fernandez. 
Raymond Martinez Fernandez (December 17, 1914  March 8, 1951) and Martha Jule Beck (May 6, 1920  March 8, 1951) were an American serial killer couple. 
