(CBS News) The head of the U.S. Anti-Doping Agency, Travis Tygart, is speaking out for the first time since the agency stripped seven-time Tour de France winner Lance Armstrong of all of his titles.
Tygart says Armstrong has been doping since his first win at the Tour de France in 1999 and went on to intimidate witnesses throughout the doping investigation, to the extent that they were "scared of the repercussions of simply telling the truth."
In a statement, Armstrong's lawyer refutes Tygart's claim that Armstrong offered a "significant financial donation" to USADA, a donation Tygart called "an inherent conflict of interest" that he had "no hesitation in rejecting."
Tygart spoke to CBS New's Scott Pelley about the cycling star's drug testing history and Armstrong's hand in intimidating witnesses.
TYGART: Six samples that were taken from Lance Armstrong were retested in '05 and they were positive.
PELLEY: In '99 when the tests were originally taken, was it reported that they were negative?
TYGART: There was no test for EPO (the performance-enhancing drug erythropoietin). They were not tested for EPO at that time.
PELLEY: And when you tested for them in 2005, you discovered that they were --
TYGART: All six were flaming positive.
PELLEY: Flaming positive?
TYGART: Flaming positive.
Tygart told Pelley that throughout the investigation, witnesses were intimidated to try to keep the code of silence from breaking.
PELLEY: Was Lance Armstrong personally involved in intimidating these other riders to keep them quiet?
TYGART: He was. It was tough. All -- all these witnesses were -- were scared of the repercussions of them simply telling the truth.
PELLEY: What could Lance Armstrong do to them?
TYGART: Incinerate them.
Former teammate Levi Leipheimer felt the heat. In his sworn affidavit, he says he came to a cycling dinner after he testified to the grand jury. Leipheimer says Armstrong was there and sent Leipheimer's wife a text that read, "Run don't walk."
PELLEY: What did she take it to mean?
TYGART: It's a veiled threat. Knowing her husband had just testified, truthfully, in front of the grand jury and had told citizens of this country about this great fraud. It was a message: You better run.
PELLEY: Your investigation showed that there were personal threats made against riders who had decided to come clean. I wonder if there were any threats against you.
TYGART: There were, Scott.
PELLEY: These threats came from where?
TYGART: Emails, letters.
PELLEY: Can you remember any of the lines from the emails or the letters?
TYGART: The worst was probably putting a bullet in my head.
PELLEY: Did you take that seriously?
Tygart believes Armstrong had undue influence over the International Cycling Union.
PELLEY: Lance Armstrong made a generous donation to the International Cycling Union of $100,000. Do you think that was made to influence them?
TYGART: I don't know. Obviously totally inappropriate.
PELLEY: Why inappropriate? Lance Armstrong is trying to support anti-doping in the sport. That's what he would tell you.
TYGART: It's an inherent conflict of interest.
A conflict that Tygart also faced. He told us for the first time in this interview that Armstrong tried to make a donation to the U.S. Anti-Doping Agency in 2004.
PELLEY: What kind of donation was he interested in making to USADA?
TYGART: It was a significant financial donation.
PELLEY: Which came to what?
TYGART: One of his representatives made an offer to us. It was well in excess of $150,000.
PELLEY: More like a quarter of a million dollars?
TYGART: It was around that ballpark.
PELLEY: When you heard that, what did you think?
TYGART: I was stunned.
PELLEY: Did you feel like you were being bought off?
TYGART: It was clear conflict of interest for USADA. We had no hesitation in rejecting that offer.
To hear the rest of Travis Tygart's story, tune into the premiere edition of "60 Minutes Sports" tomorrow at 10:00 p.m. on the Showtime Network.