The head of the World Anti-Doping Agency has dismissed Lance Armstrong's claim that he did not take banned substances after 2005.
The disgraced American admitted using performance-enhancing drugs during each of his seven Tour de France victories, but he insisted he took nothing illegal when he made his comeback in 2009 after three years out of the sport.
But WADA president John Fahey said: "The evidence from the United States Anti-Doping Agency is that Armstrong's blood tests show variations in his blood that show with absolute certainty he was doping after 2005.
"Believe USADA or believe Armstrong? I know who to believe.
"It struck me that the statute of limitations under US law might be relevant and Armstrong would not want to admit to anything in regards to his comeback that might be picked up under the US criminal code."
Armstrong also insisted in his interview with Oprah Winfrey that he would be "the first man through the door" if he was asked to give evidence to the USDA under oath.
But Fahey added: "The USADA invited him to come clean and advised he would have to give evidence under oath and provide substantial assistance.
"If he indicated the nature of the evidence - and he would have to name times, dates, people - there may be a consideration of reducing his life sentence to a term of years.
"But he never came back, he went to Oprah instead and that indicates how sincere he really was. He wanted to control the way his story was told.
"This bloke is a cheat and did my view of him change after watching the interview? No."