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updated 1:32 a.m. ET Jan. 26, 2013

(Reuters) - The United States Anti-Doping Agency (USADA) has set disgraced cyclist Lance Armstrong a February 6 deadline to fully cooperate in the investigation into cycling's darkest episode in return for a possible reduction of his life ban.

In excerpts of his interview with the CBS network scheduled for Sunday broadcast, USADA CEO Travis Tygart said he had written a letter to Armstrong with the offer.

Armstrong, 41, admitted in an interview with Oprah Winfrey last week that he had cheated his way to a record seven Tour de France titles with systematic use of banned, performance-enhancing drugs. Last year he was stripped of his titles after being labeled a "serial cheat" by the USADA.

Tygart also disputed Armstrong's claims of a clean comeback in 2009.

"His blood tests in 2009, 2010 ... one to a million chance that it was due to something other than doping," Tygart said.

(Reporting by Amlan Chakraborty in New Delhi.; Editing by Patrick Johnston)