On the evening of December 7, 1993, a rush-hour train ride from New York City to Long Island erupted in violence. Without provocation or apparent motive, a black man suddenly began shooting his semiautomatic pistol at white and Asian passengers as he walked up and down the aisle on the crowded train. Three passengers subdued the man as he paused to reload his weapon. Six people were killed, and over 20 others were wounded. Colin Ferguson, a 36-year-old man, was held in the shooting. When it was over, he simply said, “I've done a bad thing.”

Ferguson, a native of Kingston, Jamaica, was a single, unemployed man living in Brooklyn, New York at the time of the murders. After his arrest, police found notes in his ...