| [Source] | |
"Mugsy" Tannen was a Chicago gang boss in 1927, he was the son of Buford "Mad Dog" Tannen, and the grandfather of Biff Tannen. In Back to the Future Part II it is seen that his wife, Gertrude Tannen, lives alone with their grandson, indicating that he died some time before 1955, most likely it weren't natural causes that killed him as he was a gang boss.