WOODRUFF (on-screen): When do you think he changed?
NADER HASAN: I don't know.
WOODRUFF: This is a very different person than the one you describe.
NADER HASAN: Doesn't -- doesn't make it any better. I mean, he did what he did now. And -- and we've lost him.
WOODRUFF (voice-over): The image of a deranged shooter is a long way from the childhood Nader remembers with Nidal, growing up together in suburban Virginia.
NADER HASAN: Two kids growing up in Arlington County, at the local fire department, meeting their firemen, putting on the firemen's hats on their heads, and thinking that they were on top of the world.
WOODRUFF: Nader says they had the typical upbringing, from birthday parties to Santa at Christmas. Graduations were family occasions. Nidal, there on the left, celebrated with Nader. They didn't speak Arabic and weren't very religious.
(on-screen): Was Nidal religious, more religious than you? Or the same?
NADER HASAN: No, not at all, same.
WOODRUFF: Same thing?
NADER HASAN: Kid, play soccer, catch fireflies, you know? And, no, we were never -- we were fast. That was the big thing.
WOODRUFF: Nidal had joined the Army out of high school and turned to religion after the death of his mother in early 2001.
NADER HASAN: That was his mom's wish, know God. And so he started praying more and becoming more pious. And then, all of a sudden, four months later, September 11th happens. Now that you might see that as your first challenge as to, how much do you believe in your faith? Who knows what was going on in his head?
WOODRUFF (voice-over): As an Army psychiatrist, Nidal was assigned to Walter Reed Hospital to counsel returning combat soldiers. His family says their traumatic stories deeply affected him. And as he became more religious, he began to question the war on terror as a war on his faith, dreading his own deployment.
He even gave PowerPoint presentation to military colleagues which seemed to solidify his evolution of beliefs. He wrote, "It's getting harder and harder for Muslims in the service to morally justify being in a military that seems constantly engaged against fellow Muslims."
NADER HASAN: There was this issue of choosing God and country. And I think that's where his sickness really started to morph.
WOODRUFF (on-screen): Do you think that Al Qaida terrorists are the ones that influenced to the point where he was ready to commit murder?
NADER HASAN: I don't know. I believe that maybe some of the things that are seen on the Internet, some of the websites -- I'm still not privy to any of it, the alleged, you know, e-mails between him and Anwar Awlaki.
WOODRUFF (voice-over): It is believed Nidal exchanged e-mails with al-Awlaki, the suspect Al Qaida terrorist in Yemen, reportedly writing to him, "I can't wait to join you in the afterlife," and asking, "When is jihad appropriate?"
(on-screen): You know, the Senate investigation called this a ticking time bomb, I guess potentially more violent as time moved on. You saw nothing like that?
NADER HASAN: Nope.
WOODRUFF: If you had known what was possibly going to happen, would you have turned him in?
NADER HASAN: Absolutely, without question, without question. And that's why we had the FBI come to our house right away, if there was anybody else out there that we could help.