Adam Frankel was trying to find out more about his family's history and his parents' divorce when he learned a secret that made him question his entire identity.
Frankel, a former speechwriter for President Barack Obama, was 25 when he learned in 2006 that his dad was not his biological father.
"When you lived your whole life for 25 years thinking you're one person, and then get a piece of information like that, that totally upends it,'' Frankel said on the 3rd hour of TODAY Monday. "It made me question everything."
Frankel, 38, details his painful journey and the trauma his family has endured in his new book, "The Survivors: A Story of War, Inheritance, and Healing."
The grandson of Holocaust survivors was seeking answers to why his parents got divorced when his mother confessed that she had an affair.
She also said his dad did not know that he wasn't Frankel's biological father because telling him would break his heart.
"I felt like I'd lived a lie my whole life, and I felt like I was living a lie, and that I was complicit in the lie,'' Frankel said. "You know, every time I'd see my dad, every time I'd see my grandmother, I felt like I was withholding something from them, something profound and personal."
He found out that his biological father was one of his mother's graduate school professors, a man he had known his entire life as his mother's friend.
However, it took him eight more years before he told the man who raised him the truth.
"Carrying the secret was so hard, and it was just so disruptive to my life, so painful,'' he said.
Frankel had lumps under his arms from stress, chronic panic attacks, sleeplessness and nightmares, all from holding the secret inside.
"I sensed that opening up to him would be the path to help me unlock this and move forward,'' he said.
He told his dad the truth in 2014, which had a profound impact on Frankel's life.
"He said, 'I know. I've always known it was possible, and I made a decision a long time ago that it doesn't matter one way or another — you're my son and you always will be,''' Frankel said.
Frankel currently does not have a relationship with his biological father, who had no comment when contacted by TODAY.
Frankel has since made peace with his mother.
"I have forgiven my mom,'' he said. "Writing this book helped me explore and understand my mom's mental health issues which she's battled with all of her life, helped me understand some of the struggles she's had, helped me understand my family's story.
"And all of that helped me empathize and sympathize with her, and I do forgive her."
He also wishes he hadn't held on to the secret for so long.
"In my case, my biggest regret about this whole thing is not telling my dad sooner,'' he said. "... My fear was that he would look at me slightly differently."
He added, "I think one of the lessons that I've learned from this whole thing is that family is not built on biology, it's built on love."