Two years ago, Saudia Muwwakkil, the director of communications for the Martin Luther King Jr. National Historic Site, invited community leaders to discuss how to mark the 100th anniversary of a 1906 race riot in which mobs of whites descended on the city's black residents. The racial strife shut down Atlanta for four days and ended with the bodies of black men hanging from trees and streetlights. But of those Ms. Muwwakkil called, almost none had heard of it.