When a young Southern Baptist pastor named Alan Cross arrived in Montgomery, Ala., in January 2000, he knew it was where the Rev. Martin Luther King, Jr. had his first church and where Rosa Parks helped launched the famous bus boycott, but he didn’t know some other details of the city’s role in civil rights history.

The more he learned, the more troubled he became by one event in particular: the savage attack in May 1961 on a busload of Black and white Freedom Riders who had traveled defiantly together to Montgomery in a challenge to segregation. Over the next 15 years, Cross, who is white, would regularly take people to the old Greyhound depot in Montgomery to highlight what happened that spring day.