David McCullough was a young researcher at the U.S. Information Agency when he walked into the Library of Congress in 1961 and chanced upon a photography exhibit depicting the 1889 flood in Johnstown, Pa., the deadliest in American history.

“I was overwhelmed by the violence revealed in them, the destruction,” Mr. McCullough, who was from the same area of western Pennsylvania, later told the Johnstown Tribune-Democrat. More than 2,200 people died, and a thriving coal-and-steel town was submerged in muddy debris because wealthy industrialists had neglected a dam.

The passage of time had reduced the tragedy to a historical footnote, Mr. McCullough discovered, with little if any serious scholarly study devoted to it. Undaunted by his own inexperience — “I imagined myself being a writer, but never a writer of history,” he said — he set out to write a book about the Johnstown flood.

For years, he dedicated his spare time to his research, interviewing the few remaining survivors to capture their memories of sudden terror, desperate acts of self-preservation and the awful duty, in the aftermath, of identifying the dead.

“The Johnstown Flood,” published in 1968, became a bestseller, rekindled national interest in the disaster and instantly established its author as a historian with an exceptional gift for animating history.

Mr. McCullough, long regarded as a master storyteller of American daring, endeavor and perseverance, died Aug. 7 at his home in Hingham, Mass. He was 89.