James "Big Jim" Colosimo (born Giacomo Colosimo) was an Italian-American mob crime boss who built a criminal empire in Chicago based on prostitution, gambling, and racketeering. Immigrating from Italy at the end of the 19th century, he gained power through petty crime and the heading of a chain of brothels.

From about 1902 until his death in 1920, he would lead a gang that would be known after his death as the Chicago Outfit . Johnny Torrio, an enforcer Colosimo imported in 1909 from New York, seized control after his death. It was alleged that Al Capone, a Torrio henchman, was directly involved in the murder.

http://www.youtube.com/watch?v=6GRnkys0OjM&list=PLQhDH_WZbs1N0jqZ9y9naMtgUfZ3VoVbo

From what I have heard it was his reluctance to get involved with prohibition is what lead to him getting whacked.

John Kobler, in his excellent bio of Capone, implies but doesn't state that Torrio arranged the murder because Colosimo wasn't paying attention to the potential of Prohibition. Laurance Bergreen, in a later, excellent bio of Capone, says Frankie Yale did it to expand his influence in Chicago. But he never follows up on that idea.