What was the cause of Earl Little's death in the book, X: A Novel?

Earl Little, Malcolm's father, was pushed in front of a streetcar by members of the Ku Klux Klan when Malcolm was six years old. This was a defining moment in Malcolm's life, and his father is a constant spectral presence during his coming-of-age. Malcolm always wonders what his father would think of his actions, and usually finds himself feeling ashamed as a result.