elizabeth palmer, cbs news. >>> back in this country, the fiercely competitive world of professional cycling is being shaken again by accusations of doping aimed at lance armstrong. and disgraced tour de france winner floyd landis is the accuser. sandra hughes has more on that from los angeles. >> reporter: after crashing on the tour de california, lance armstrong got back on his bike. but his injuries finally forced him to quit just hours after he vehemently denied the latest allegations of doping. the accusations against armstrong, who has won the tour de france seven times, came in e-mails from floyd landis. landis, who won the tour de france in 2006, tested positive for performance enhancing drugs and was stripped of his victory. he spent four years fighting the allegations. >> did you use testosterone at all? >> no. >> reporter: then he abruptly put the brakes on his defense. landis, according to the "wall street journal," sent e-mails to u.s. and world cycling officials admitting to using the blood booster epo to increase his endurance, testosterone and human growth hormone. he