When Spider Martin, a young photographer for The Birmingham News, stepped onto the Edmund Pettus Bridge in Selma, Ala., on March 7, 1965, he knew exactly what to do.

He ran to the top of the bridge “and got myself situated, like I’d done so many times, like shooting a football game, staying 10 or 20 yards ahead of the action, never knowing what the score was,” he later recalled.

Today, everyone knows the score from that day in Selma, known as Bloody Sunday, thanks in part to Mr. Martin’s powerful images of the police beating back peaceful civil rights marchers, which were blasted around the world via The Associated Press.