Federal prosecutors have intensified their criminal investigation of the cyclist Lance Armstrong since the end of the Tour de France last month. They questioned many of his former associates, including cyclists who have supported and detailed claims that Armstrong and his former United States Postal Service team participated in systematic doping, according to a cyclist who has been interviewed and two others privy to the inquiry.

In May, Armstrong’s former teammate Floyd Landis shook the cycling world by publicly accusing Armstrong and other team members of using performance-enhancing drugs and blood transfusions to gain an unfair advantage. Landis said that Armstrong  the biggest name in the sport  had encouraged doping and that the team had sold its bikes to help finance an expensive doping program.

Armstrong has denied any wrongdoing and has said that Landis, who was stripped of his 2006 Tour title for doping and received a two-year ban from the sport, has no credibility.

But now, prosecutors and investigators have more than Landis’s account to go on, according to the two people with knowledge of the investigation. They spoke on condition of anonymity because they did not want to jeopardize their access to sensitive information.

A former teammate of Armstrong said in a telephone interview Wednesday that he had spoken with investigators. He said he detailed some of his own drug use, as well as the widespread cheating that he said went on as part of the Postal Service team  all of which he said was done with Armstrong’s knowledge and encouragement.

The rider, who has never tested positive for performance-enhancing drugs or methods, asked that his name not be used because investigators advised him not to speak publicly about the information he provided. He has not been called before the grand jury that has been convened in Los Angeles to investigate the case.

Riders have been compelled to come forward. Tyler Hamilton, who is serving an eight-year ban for using performance-enhancing drugs, has met with the grand jury, those who have been briefed on the case said. His lawyer, Chris Manderson, said that Hamilton had received a grand jury subpoena but did not say whether Hamilton had already provided testimony.

Armstrong is considered one of the more remarkable athletes in American history, someone who dominated his sport and also had a compelling personal story, having beaten testicular cancer.

Jeff Novitzky, a special agent for the Food and Drug Administration, is in charge of the investigation and has been interviewing Armstrong’s associates and former teammates.

Novitzky, the lead investigator in the Bay Area Laboratory Co-operative steroids case, is trying to determine if Armstrong, his teammates, the owners or managers of his former team conspired to defraud their sponsors by doping to improve their performance and win more money and prizes.

Armstrong did not respond to requests for comment put directly to his agent and his manager on Wednesday.