JEFFREY BROWN: And next: A new book takes a look at the roots of the first lady's family tree.

Gwen Ifill has that story.

GWEN IFILL: Among the four million slaves living in the United States on the eve of the Civil War, there was a 10-year-old girl who, a century-and-a-half later, would turn out to be the third-great-grandmother of Michelle Obama.

Even Mrs. Obama didn't know this family history until New York Times reporter Rachel Swarns unearthed her legacy in 2009. The first lady's ancestry, both black and white, are a complicated heritage shared by many Americans.

"American Tapestry," the story of the black, white, and multiracial ancestors of Michelle Obama, takes us on that journey. Its author, Rachel Swarns, joins me now.

Rachel, thanks for joining us.

You started out after having written about the story for The New York Times to find out about the genealogy of Michelle Obama, and instead you found the story of American history.

RACHEL SWARNS, author of "American Tapestry": That's right.

It really is the sweep of the country's history through the lens of one family, this first lady's family.

GWEN IFILL: A family that turned out to be not necessarily—you traced it backwards from knowing, of course, who—where it ended, but tracing it backward was kind of the drama.

RACHEL SWARNS: Right.

Well, the first lady has always known that she had white ancestors, but she didn't know who or when or where. And so I wanted to take the reader back into time to try and solve that mystery.

GWEN IFILL: Who was Melvinia?

RACHEL SWARNS: Melvinia was a slave girl valued at $475 in 1852, and she was the first lady's great-great-great-grandmother.

And she ended up going from a farm in Spartanburg, S.C., to Georgia, where she fathered a child, a biracial child. And the question has been, who was the father of that child?

GWEN IFILL: And so you set out to figure that out. But how do you trace that sort of thing?

RACHEL SWARNS: It's very challenging.

I mean, just telling these stories are challenging, particularly for African-Americans, because Melvinia was unusual. She appeared in a will. But before the Civil War, people simply didn't appear. African-Americans didn't appear in the census, and their marriages and births weren't chronicled in newspapers. So it's not easy.

GWEN IFILL: I noticed throughout the book you often had to fall into kind of a "this may have happened" construction. That must have been kind of frustrating for a reporter.

RACHEL SWARNS: It is. And the reality is that there are some things we just won't know.

GWEN IFILL: Was the path that you took—the path that they took, that this family took, was it typical? Was it—how widespread was it?

RACHEL SWARNS: She—her family story is very, very typical.

It is the story of so many Americans. And they basically had front-row seats to major moments in our history, from slavery to the Civil War, Reconstruction, segregation, the migration. It is a very, very American story.

GWEN IFILL: You—there was a lot written about when this book originally came out about Michelle Obama's white ancestors, even when you first wrote the story for The New York Times.

RACHEL SWARNS: That's right.

GWEN IFILL: How unusual was that, really? We can look now at the African-American experience and see it's a rainbow, as much as anything else.

RACHEL SWARNS: It is a rainbow. And many of us have those stories, and many people are finding that out through DNA testing themselves.

You know, with genealogy tools available online, with a cheek swab, and off it goes in the mail, a lot of ordinary people are finding these stories out and making these kinds of connections.

GWEN IFILL: But it raises lots of uncomfortable questions, too, especially about how the original connection happened.

RACHEL SWARNS: It does.

And I was able to find the mystery white ancestors in her family tree, and their descendants. And they, as you might imagine, really grappled with this. It's hard to look back and to know that your family may have owned the first lady's family, in fact, indeed did own the first lady's family, and, worse still, that your ancestor may have raped a member of the first lady's family. These are not easy things to think about.

GWEN IFILL: And there's really no way to tell, in the kind of research you did, what the nature of the relationship was between Melvinia and the man who fathered her child.

RACHEL SWARNS: Right. There's no way to know.

GWEN IFILL: And Dolphus Shields, he's a key character in this. Tell me about him.

RACHEL SWARNS: So he is the first lady's great-great-grandfather. He was biracial, born a slave.

And he really carried the family forward. He became a carpenter. He became a property owner. He became—he ran his own business. He founded churches. When he died, his obituary ran on the front page of the black newspaper in Birmingham at the time.

GWEN IFILL: And we think that he had a relationship, perhaps, with his white father, even if he didn't know it was his father?

RACHEL SWARNS: Well, we don't know. There are intriguing questions about that.

He left Georgia for Birmingham. And around the time he was living in Birmingham, his—he had a white half-brother who also lived in Birmingham. And there are people who knew Dolphus who said that he talked about having a white brother. Whether that really was this half-brother, whether he knew who his father really was, we don't know.

GWEN IFILL: In putting this all together, in knitting this all together, did you talk to current-day members, descendants of this—of this tree?

RACHEL SWARNS: Yes, I talked to members black and white. Some of them actually got together quite recently.

GWEN IFILL: Tell me about that.

RACHEL SWARNS: Yes.

They—the town where Melvinia once lived as a slave decided to erect a monument to Melvinia after the story that appeared in the front page of The New York Times. And they had a ceremony. Some of Melvinia's descendants were there.

And, at the last minute, I thought maybe some of the white descendants would like to come. And they did. Some drove from Birmingham and parts of Alabama. And some came from Georgia.

GWEN IFILL: Wow.

RACHEL SWARNS: It was quite something to see.

GWEN IFILL: I will bet. I will bet it was.

Yet, along the way, there has always been a certain amount of shame and secret-keeping that goes with this kind of connection. And I want you to read a passage from the book that I asked you to take a look at that kind of captures—at least in reading it, it captured it for me.

RACHEL SWARNS: "That reluctance to probe the past, to look back over one's shoulder, to examine the half-healed sores that festered in grandparents and great-grandparents reappears over and over again in Mrs. Obama's family tree.

"It has made the search for the truth that much harder. But it is also understandable.

"People often turn away from what is too painful to witness. They almost always want their children to see the world as a better place, to be free of their pain."

GWEN IFILL: In meeting with the descendants, as they met each other for the first time recently, did it seem as if they had transcended that pain?

RACHEL SWARNS: I think they were willing to grapple with it.

And I—I think, in many ways, they would have wished that this connection might have originated in a different way, but they accepted it and thought that they, as contemporary people, could get to know each other and exchange phone numbers, take a picture, have a dinner.

GWEN IFILL: And do you know if other African-Americans and whites who have grown together and grown apart in our society have also found their way back to each other in this way?

RACHEL SWARNS: Oh, many, many people are doing this all the time.

And when you do these DNA tests, they connect you to your distant cousins. And for many African-Americans, they find they're black, white, and in between.

GWEN IFILL: Fascinating.

Rachel Swarns, author of "American Tapestry," thank you so much.

RACHEL SWARNS: Thank you.