"The Last of the Hitlers" author David Gardner

(CNN) -- Adolf Hitler left no offspring when he died in his bunker in 1945. But he wasn't the last of the Hitler line. He had a nephew, William Patrick Hitler, who grew up in England, moved to America, and had three sons.

The story of those Hitlers is told in a new documentary, "The Last of the Hitlers," based on the book of the same name by British journalist David Gardner. CNN's Paula Zahn spoke to Gardner on Tuesday's "American Morning."

PAULA ZAHN, CNN ANCHOR: Very remarkable story to share with you now. There are three brothers who live normal, anonymous lives with their mother in a modest house in Long Island, New York. But the three men have a darkly unique heritage. They happen to be the great nephews of Adolf Hitler, the last known living relatives of the murderous tyrant.

A new documentary, called "The Last of the Hitlers," tells the story of the brothers, and their bizarre pact with each other never to have children in order to sever the bloodline of their infamous relative.

The film is based on the fascinating book of the same name by journalist David Gardner, who joins us now from Los Angeles, California.

So David, how did you find these brothers?

Photos of Hitler's great nephews that appear in Gardner's book

DAVID GARDNER, AUTHOR, "THE LAST OF THE HITLERS": Well, it was a long journey. About 1995, I was working with a news agency in New York, and I was asked to try and track them down, track down William Patrick Hitler, who was Hitler's nephew. There had been some cuttings, old newspaper cuttings, from before the Second World War, and that's pretty much the last anyone heard of him.

So, I kind of started with a phone book, looking under Hitler in the phone book. Didn't get very far. It was a long journey. Took me about four years to find the family.

ZAHN: And once you found them, what proof did you have that these men were actually related to Hitler?

GARDNER: Well, for a start, I had birth dates, and documentary evidence before I actually approached the family, and then when I actually knocked at their door, this is the first time anyone had actually knocked at their door for 50 years. So it was something of a shock to them, but William Patrick's widow confirmed that her husband was indeed -- or had indeed -- been the nephew of Adolf Hitler.

ZAHN: Sorry -- sorry. I was just going to refer back to the head shots we just saw. There didn't seem to be any overt physical resemblance to Adolf Hitler. What were the similarities you found, if any, between these nephews and their uncle?

GARDNER: Well, I think that is the point. Apart from a very vague resemblance in looks, these -- this part of the family is so far removed from Adolf. They've lived all-American lives. They live in a small town in Long Island. ... They were born in America, and these are the American Hitlers, in effect.

But they've lived very different lives to the one that the Fuhrer lived, and indeed, a different life than then one their father lived. Their father actually grew up in England, spent six, seven years in Germany in the 1930s, where his uncle gave him a job, and then he came to America just before the Second World War, and the family's been here ever since.

ZAHN: Tell us a little bit about this blackmail letter that you learned of. Who had the letter and what did it tell us?

GARDNER: Well, William Patrick, as I said, was working in Germany in the '30s, and he'd gone there hoping to benefit from his uncle's position. At that time, having a Hitler in Germany, there was a good chance he was going to get a good position, but he found that he was kind of knocked around -- he worked at a lowly bank job, he worked in a car factory, never really getting any decent money or any position. He sent a blackmail letter to Adolf, basically saying: If you don't give me a better job and treat me a little bit better, I'll go public with the speculation within the family that Hitler himself had a Jewish grandfather.

ZAHN: Whatever became of that threat?

GARDNER: Well, in fact, Hitler kind of bowed down to it, this lowly nephew, and did give him some money, which is kind of curious. I mean, of all the terrible things that Hitler did, the one person that stood up to him seems to have been his own nephew, and who went away with the equivalent now to a quarter-million dollars.

ZAHN: Let's talk about the reality of the lives these nephews live. Did they all change their names so they could live in relative obscurity here?

GARDNER: Yeah, that's the case. In fact, when William Patrick, Adolf's nephew, came to America, he went on to serve in the U.S. Navy and fought against his uncle. But after the war, obviously, it became clear that having the name Hitler was not a good thing to have. And he changed his name, and went on to have -- to marry, have a family, and they lived in total anonymity. That was for the last 50 years.

ZAHN: And David, is it true the nephews signed a pact making the agreement that none of them would ever bear children so that the bloodline would basically stop with them?

GARDNER: They didn't sign a pact, but what they did is, they talked amongst themselves, talked about the burden they've had in the background of their lives, and decided that none of them would marry, none of them would have children. And that's something that -- a pact they've kept to this day.

ZAHN: Well, it's amazing, it took you four years to find them. The story is absolutely fascinating, as is the book. We very much appreciate your getting up at this ungodly hour on the West Coast to join us this morning.

GARDNER: It's a pleasure. Thank you.

ZAHN: David Gardner, thank you very much for your time this morning.