Cause of death: heart attack
Eliot Ness (April 19, 1903 – May 16, 1957) was an American Prohibition agent known for his efforts to bring down Al Capone while enforcing Prohibition in Chicago. He was leader of a team of law enforcement agents nicknamed The Untouchables, handpicked for their incorruptibility. The release of his memoir The Untouchables, months after his death, launched several screen portrayals establishing a posthumous fame for Ness as an incorruptible crime fighter.
Source : Wikipedia