Sixty-six-year-old Andrés Román Díaz walked home from his job at a nursery in northern San Diego County carrying three gallons of water and two bags of groceries. When he reached the canyon where he lived in a makeshift hut, a white car drove past and someone shot an air rifle at him. When several teenagers got out of the car and started to follow him, Díaz dropped his bags and started running. Behind him, he could hear the boys taunt, “Go back to Mexico!” Before the night was over on July 5, 2000, Diaz and four other elderly migrant workers were shot with air rifles and beaten with rocks and metal pipes by eight high school students.

Just a hundred miles away from the canyon where Díaz lives, volunteers...