Armstrong could face a lifetime ban from the sport if he is found to have used performance-enhancing drugs. The move by USADA (yoo-SAH'-duh) immediately bans him from competing in triathlons, which he turned to after he retired from cycling last year.

In a strong denial, Armstrong called the doping allegations "baseless" and "motivated by spite" and noted they came just months after federal prosecutors closed a two-year criminal investigation against the cyclist without bringing an indictment.