US prosecutors stepped up their criminal probe of cyclist Lance Armstrong on allegations of doping after last month's Tour de France, the dpa reported reffering to the New York Times report on Thursday.
Investigators have been questioning former associates who have given details on claims that Armstrong and his former United States Postal Service team participated in systematic doping, the reported quoted three unnamed cyclists as saying.
The US federal probe began after another US rider, Floyd Landis, who was stripped of the 2006 Tour title for failing a doping test, accused Armstrong and others.
Last month, former Tour de France champion Greg LeMond, a long-time rival, repeated doping accusations against Armstrong.
In March, Armstrong's RadioShack teammate, Li Fuyu, tested positive for the banned substance Clenbuterol following an in-competition test during the Dwars Door Vlaanderen race in Belgium.
Armstrong has denied any wrongdoing.
The Times reported that the investigation is being handled by Jeff Novitzky, a special agent for the US Food and Drug Administration who was the lead investigator in the Bay Area Laboratory Co-operative (BALCO) steroids case that brought down American running great Marion Jones.
Armstrong won the Tour de France a record seven consecutive times after surviving testicular cancer that had spread to other parts of his body