Lance Armstrong lied in his confessional interview with Oprah Winfrey and the shamed cyclist has two weeks to finally come clean, the US anti-doping official who pursued him for years has said. Travis Tygart said in an excerpt of an interview with the CBS network that Armstrong failed to tell Winfrey the truth about several key points over doping - including a claim that he raced drug-free in his comebacks in 2009 and 2010. Tygart, the US Anti-Doping Agency (Usada) chief, said he had written to Armstrong to say that if he wanted to lessen his lifetime sporting ban he must "co-operate fully and truthfully" by February 6, about drug-taking in the sport. It is not clear if co-operation from Armstrong, who was stripped of all seven of his Tour de France wins last year, could take the form of testimony in front of a truth and reconciliation commission for the sport. The International Cycling Union (UCI), which is under pressure from the World Anti-Doping Agency and the Usada, has agreed that such a platform would benefit the drug-damaged sport after a series of devastating doping cases. Armstrong, a cancer survivor who during the Oprah interview admitted doping for the first time after years of vehement denials, said he would be willing to testify before such a commission if he were invited. He also said that his record seven consecutive wins in the tour - 1999-2005 - were fuelled by performance-enhancing drugs, but insisted he was clean when he came out of retirement and raced in the Tour de France in 2009 and 2010. Tygart, however, in the CBS interview which was to air overnight on 60 Minutes , said the latter claim was "just contrary to the evidence". According to Tygart, expert reports based on the variation of Armstrong's blood values in 2009 and 2010 make it a "one to a million chance that it was due to something other than doping". The Usada chief reiterated the assertions in the report issued last year by the agency on which it based its lifetime ban of Armstrong and the forfeiture of all his cycling results from August 1998. The report led to Armstrong's demise after more than a decade of denials that he was a drug cheat, during which he pursued a series of vitriolic attacks against several individuals who had accused him of doping. Tygart told CBS that Armstrong may have lied about doping after his comeback because under the statute of limitations for criminal fraud, he would still be open to prosecution. He also took issue with Armstrong's claim that the disgraced Texan's favoured drug cocktail of blood-boosting EPO, blood transfusions and testosterone included just a small amount of EPO. "He used a lot of EPO," Tygart told 60 Minutes , alleging that Armstrong was less than truthful when he told Winfrey that he had not pushed his teammates toward cheating. "He was the boss," Tygart said in the excerpt. "The evidence is clear he was one of the ringleaders of this conspiracy that pulled off this grand heist that ... using tens of millions of taxpayer dollars defrauded millions of sports fans and his fellow competitors." In the second segment of his interview with Winfrey, the 41-year-old Armstrong said he wanted to compete again in sport - perhaps marathons. Immediately after Armstrong's first confession aired last week Tygart responded by saying that the former cyclist must testify under oath to have any hope of reducing his sanction.