The U.S. Anti-Doping Agency told Armstrong earlier this month that it plans to bring charges against him. If found guilty, the titles he won from 1999 to 2005 could be taken from him. Armstrong says he is innocent.
The case will be sent to a USADA review panel to determine whether there's enough evidence to warrant formal charges. If it does, Armstrong could face an arbitration panel to decide the matter by November.
In an 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."
The Associated Press contributed to this report.
