"USADA only initiates matters supported by the evidence," Tygart said. "We do not choose whether or not we do our job based on outside pressures, intimidation or for any reason other than the evidence." All named individuals are presumed innocent until proven otherwise, he added.

His statement did not mention anything about Armstrong's status as a triathlete.

According to its website, the quasi-government agency is recognized as the official anti-doping agency for Olympic, Pan American and Paralympic events in the United States.

Armstrong said the organization has "self-written rules" and punishes first and adjudicates the facts later.

Armstrong has been dogged by allegations of drug abuse in recent years, with compatriot Floyd Landis -- who was found guilty of doping in the 2006 Tour de France, resulting in him being stripped of the title -- making a series of claims last year.

Armstrong came out fighting once again in May 2011 in the face of fresh allegations made on the CBS News "60 Minutes" show by another American, Tyler Hamilton.

In the CBS interview, Hamilton -- who retired in 2009 after twice testing positive himself -- says he first saw Armstrong use blood boosting EPO in 1999, the year of his first Tour de France victory.

"I saw it in his refrigerator," Hamilton told the American news program. "I saw him inject it more than one time like we all did, like I did many, many times."

On his Twitter page, Armstrong at the time said he never failed a drug test.