The agency said Armstrong used performance-enhancing drugs and other improper methods to win cycling's premiere event, the Tour de France, from 1999-2005. Friday was the deadline for Armstrong to respond to USADA's warning that charges were pending before his case moves to the next stage.
Armstrong, who denies doping and noted he has never failed a drug test, could be stripped of his titles and banned from cycling, though he retired from the sport last year.
In their 11-page document, Armstrong's attorneys complained they still haven't been allowed to see the evidence against him, including witness names and any expert analysis to support USADA's claim that 2009 and 2010 blood tests are "fully consistent" with blood doping.
The letter said USADA's case is "long on stale allegations disproved long ago and short on evidence" and "offensive to any notions of due process."
Armstrong's case now goes to a three-person USADA review board, which will decide if there is enough evidence to support the charges. If USADA files formal charges, the case could go to a three-person arbitration panel by November. Armstrong's letter urged the review board to reject the case.
"The Review Board must recommend that this case not move forward," the letter said.
Armstrong's attorneys made similar claims in previous letters, but Friday's document appeared to lay out his potential legal strategy should he file a federal lawsuit against USADA.
Armstrong's letter argues that USADA's rules allow the review board to consider materials submitted from an accused athlete, but complains that he can't mount a legitimate defense until he's able to see the evidence against him. USADA has said it is withholding witness identities to protect them from intimidation.
USADA's warning letter to Armstrong said 10 former teammates are willing to testify that they either know Armstrong used performance-enhancing drugs or heard him encourage their use among team members. USADA says Armstrong used the blood-booster EPO, steroids and improper blood transfusions.
Armstrong's attorneys say they believe USADA investigators coerced false testimony from witnesses by promising not to charge them with doping; they argue this could violate bribery laws.
They also question whether USADA improperly gained access to testimony in a recent federal grand jury criminal investigation that ended in February with no charges filed against Armstrong. The letter notes that USADA Chief Executive Officer Travis Tygart participated in witness interviews with federal criminal investigators.
Armstrong's letter also challenged the 2009-2010 blood tests, which were taken during his two-year comeback from retirement. Armstrong passed all his drug tests during that period and posted his testing results on his website, Livestrong.com, the letter said.
It also said most of the allegations fall outside of USADA's eight-year statute of limitations but the agency argues that Armstrong keeps expanding the time limit by continuing to deny drug use.
USADA has said it is also pursuing doping charges against Johan Bruyneel, the manager of Armstrong's winning Tour de France teams; team doctors Pedro Celaya and Luis Garcia del Moral; team trainer Pepe Marti, and consulting doctor Michele Ferrari.
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