The authorities, who are in the early stages of their investigation, are seeking to determine whether Armstrong, the owners and managers of his former cycling teams, and his teammates conspired to defraud their sponsors by doping to improve their performances and garner more money and prizes, one of the people said.

In particular, the authorities want to know whether money from the US Postal Service, the main sponsor of Armstrong’s team from 1996 to 2004, was used to buy performance-enhancing drugs, one of the people said.

The two people briefed on the investigation spoke on the condition of anonymity because they did not want to jeopardize their access to delicate information. Armstrong, through his manager, Mark Higgins, did not immediately respond to an e-mail requesting comment. Armstrong has repeatedly denied any allegations about doping.

The authorities will also closely examine the contract between Armstrong and SCA Promotions, a company that refused to pay him a $5 million bonus in 2004 after a book was published that alleged Armstrong engaged in doping, one of the people said. Armstrong sued SCA Promotions and the case — which resulted in hours of testimony by Armstrong and others under oath — was later settled out of court. SCA was forced to pay $5 million and about $2.5 million in penalties.

Federal agent Jeff Novitzky, who has been the lead investigator on the major doping investigations since the Bay Area Laboratory Co-operative case began in 2002, is playing a direct role in the investigation. Novitzky has already met face-to-face with cyclist Floyd Landis, who lives in Southern California, and has been working closely with officials from the US Anti-Doping Agency.

Meanwhile, Landis suffered another blow to his credibility. The International Cycling Union said no riders tested positive for EPO at the 2001 Tour de Suisse, disputing comments made by the disgraced cyclist.

Landis, who was stripped of the 2006 Tour de France title for doping and served a two-year ban, has claimed that Armstrong tested positive at the Tour de Suisse and officials covered it up.