Jan. 18, 2013 -- Lance Armstrong, 41, began to cry today as he described finding out his son Luke, 13, was publicly defending him from accusations that he doped during his cycling career.
Armstrong said that he knew, at that moment, that he would have to publicly admit to taking performance-enhancing drugs and having oxygen-boosting blood transfusions when competing in the Tour de France. He made those admissions to Oprah Winfrey in a two-part interview airing Thursday and tonight.
"When this all really started, I saw my son defending me, and saying, 'That's not true. What you're saying about my dad? That's not true,'" Armstrong said, tearing up during the second installment of his interview tonight. "And it almost goes to this question of, 'Why now?'
"That's when I knew I had to talk," Armstrong said. "He never asked me. He never said, 'Dad, is this true?' He trusted me."
He told Winfrey that he sat down with his children over the holidays to come clean about his drug use.
"I said, 'Listen, there's been a lot of questions about your dad, about my career and whether I doped or did not dope,'" he said he told them. "'I always denied that. I've always been ruthless and defiant about that, which is why you defended me, which makes it even sicker' I said, 'I want you to know that it's true.'"
He added that his mother was "a wreck" over the scandal.
Armstrong said that the lowest point in his fall from grace and the top of the cycling world came when his cancer charity, Livestrong, asked him to consider stepping down.
After the U.S. Anti-Doping Agency alleged in October that Armstrong doped throughout his reign as Tour de France champion, Armstrong said, his major sponsors -- including Nike, Anheuser Busch and Trek -- called one by one to end their endorsement contracts with him.
"Everybody out," he said. "Still not the most humbling moment."
Then came the call from Livestrong, the charity he founded at age 25 when he was diagnosed with testicular cancer.
"The story was getting out of control, which was my worst nightmare," he said. "I had this place in my mind that they would all leave. The one I didn't think would leave was the foundation.
"That was most humbling moment," he said.
Armstrong first stepped down as chairman of the board for the charity before being asked to end his association with the charity entirely. Livestrong is now run independently of Armstrong.
"I don't think it was 'We need you to step down,' but, 'We need you to consider stepping down for yourself,'" he said, recounting the call. "I had to think about that a lot. None of my kids, none of my friends have said, 'You're out,' and the foundation was like my sixth child. To make that decision, to step aside, that was big."
In Thursday's interview installment, the seven-time winner of the Tour de France admitted publicly for the first time that he doped throughout his career, confirming after months of angry denials the findings of the U.S. Anti-Doping Agency, which stripped him of his titles in October.
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Armstrong said in the interview that he stopped doping following his 2005 Tour de France victory and did not use banned substances when he placed third in 2009 and entered the tour again in 2010.
Investigators familiar with Armstrong's case, however, told ABC News today that Armstrong did not come completely clean to Winfrey, and that they believed he doped in 2009.
They said that Armstrong's blood values at the 2009 race showed clear blood manipulation consistent with two transfusions. Armstrong's red blood cell count suddenly went up at these points, even though the number of baby red blood cells did not.
Investigators said that was proof that he received a transfusion of mature red blood cells.
If Armstrong lied about the 2009 race, it could be to protect himself criminally, investigators said.
Federal authorities looking to prosecute criminal cases will look back at the "last overt act" in which the crime was committed, they explained. If Armstrong doped in 2005 but not 2009, the statute of limitations may have expired on potential criminal activity.
The sources noted that there is no evidence right now that a criminal investigation will be reopened. Armstrong is facing at least three civil suits.
Armstrong passed more than 500 drug tests during his career. In some cases, however, he was found to have used substances, including EPO, years after he retired when new tests could find previously untraceable drugs.
In October, the report released by the U.S. Anti-Doping Agency featured 11 of Armstrong's former teammates describing how they and Armstrong received drugs with the knowledge of their coaches and team physicians.
The U.S. Postal Service Cycling Team "ran the most sophisticated, professionalized and successful doping program that sport has ever seen," USADA said in its report.
Following the report, Armstrong was stripped of his seven Tour de France titles and lost his $75 million sponsorship deal with Nike, among others. This week, the International Olympic Committee stripped Armstrong of a 2000 Olympic bronze medal.
"It was a mythic perfect story and it wasn't true," Armstrong said of his fairytale story of overcoming testicular cancer to become one of the most celebrated cyclists in history.
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ABC News' Russell Goldman contributed to this report.