The head of the US Anti-Doping Agency has claimed shamed cyclist Lance Armstrong lied during a TV interview with Oprah Winfrey.
In an interview with CBS show 60 Minutes, which will air on Sunday, Mr Tygart said Armstrong was not telling the truth when he claimed he was clean of performance enhancing drugs when he made his comeback in 2009.
He said the 41-year-old has until February 6 to "cooperate fully" with USADA if he wants to reduce his lifetime ban from sport.
A report issued by the agency last year - on which it based Armstrong's lifetime ban and the forfeiture of all his cycling results since 1998 - found the variation of his blood values after making his comeback made it a "one to a million chance that it was due to something other than doping", Mr Tygart said.
He added that Armstrong could be motivated to lie about his comeback because under the statute of limitations for criminal fraud he would still be open to prosecution.
It comes after a lawyer for the cyclist told USADA his client will co-operate with their efforts to "clean up cycling".
Armstrong admitted last week in a televised interview with Oprah Winfrey that he took a variety of performance-enhancing drugs as he won seven Tour de France titles.
In letters sent this week between lawyers for Armstrong and the USADA, the agency's attorney William Bock asked that Armstrong testify under oath by February 6.
But Armstrong's attorney, Tim Herman, responded that Armstrong could make that deadline and called for the cycling union and the World Anti-Doping Agency to take up efforts to stamp out doping.
"As you have candidly confirmed, USADA has no authority to investigate, prosecute or otherwise involve itself with the other 95% of cycling competitors," he wrote.
"Thus, in order to achieve the goal of 'cleaning up cycling,' it must be WADA and the UCI who have overall authority to do so."
The letter said the disgraced cyclist was prepared to appear before a "truth and reconciliation" commission to be held by the International Cycling Union.
Armstrong told to Winfrey that he had used a banned "cocktail" of the blood-boosting agent EPO, blood transfusions and testosterone.
He had already been banned from cycling for life, stripped of his Tour titles and lost his Olympic bronze medal - won in the 2000 games in Sydney - after USADA found him to be a central figure in "the most sophisticated, professionalised and successful doping programme that sport has ever seen".
It is thought Armstrong's full co-operation could lead to a reduction of his ban, perhaps to eight years.