President Obama delivered a powerful rendition of "Amazing Grace" at the funeral of slain Rev. Clementa Pinckney, and called on the country to honor those killed in the South Carolina church massacre.

OBAMA: (Singing) Amazing grace, how sweet the sound... BALABAN: Musicologist Melvin Butler. BUTLER: Obama is not - I don't think he would say he's a virtuosic vocalist. But if you listen to those ...

BALABAN: Author James Walvin says it was this moment that made him want to write a book about the song. OBAMA: (Singing) Amazing grace, how sweet the sound... BALABAN: Musicologist Melvin Butler.