 Graves died of a heart attack on July 19 , 1965 while skiing in Del Norte , Colorado , twenty years after the Slotin accident , at the age of 55 . He had suffered from hypertension even before the 1946 accident , and had a minor heart attack in December 1955 . His father had also died of a heart attack . A 1978 follow @-@ up study of the Slotin accident victims suggested that latent systemic damage from the accident contributed to heart failure . He was interred in the Guaje Pines Cemetery in Los Alamos , New Mexico . 
