Fifty years ago, Senator Robert F. Kennedy climbed onto the back of a flatbed truck to deliver the news to a largely African-American crowd in Indianapolis that the Rev. Dr. Martin Luther King Jr. had been assassinated.

Kennedy was running for the Democratic nomination for president, but he scrapped his prepared remarks after hearing that King had been killed on April 4, 1968.

In their place, he delivered a largely improvised speech that might have become a footnote to history had the tumult of 1968 not soon taken his life, too.

Tensions were running high that night in Indianapolis. The first three months of the year had been turbulent, and the year before had been a violent one. Days earlier, faced with growing dissent over the war in Vietnam, President Johnson had shocked the nation with an announcement that he would not seek re-election, leaving the Democratic field open.