Hardy's "A" sample from the recent /*U.S. Olympic trials*/ tested positive, the person told The Associated Press on Wednesday night. The person requested anonymity because he was not authorized to speak publicly.
The Swimming World magazine Web site first reported the positive doping test.
Mark Schubert, head coach and general manager of the U.S. team, and Dave Salo, Hardy's personal coach at Southern California, did not immediately return phone messages left by the AP.
USA Swimming spokeswoman Jamie Olson declined to comment from the team's training camp at Palo Alto, Calif., where Hardy has departed and returned home to be with her family in Southern California, the swimmer's agent said.
Agent Evan Morgenstein told the AP that during a brief phone conversation with Hardy, she told him, "I never did anything wrong. I never cheated."
Morgenstein said he's heard there were conflicting results from Hardy's tests, though he didn't have any details.
"I'm very, very concerned about the confusion of her test coming up positive-negative-positive," Morgenstein said. "She's the one person I would never believe would do anything -- anything -- to cheat. Ever."
The Web site nbcolympics.com reported Hardy's backup "B" sample also tested positive.
If so, the 21-year-old swimmer can pursue appeals with both the American Arbitration Association and the Court of Arbitration for Sport. With the Olympics two weeks away, Hardy could appeal directly to CAS, although its ruling would be final and binding.
Typically, a first-time doping offense results in a 2-year ban.
Hardy's name was among the 596 athletes officially entered into the Beijing Games on Wednesday by the U.S. Olympic Committee.
Hardy earned spots on her first Olympic team in her best event, the 100-meter breaststroke, and the 50 freestyle and 400 free relay.
"I don't think if you had told me a month ago that I would make it in all three of these events that I would have believed you," she said at the trials. "I'm expecting good things for sure."
In Beijing, Hardy was expected to be a medal threat in the 100 breast, and to play an important part on the 400 free relay, and possibly the 400 medley relay.
If Hardy appealed and lost, the U.S. could not add to its swimming roster because the deadline to do so was July 21.
That might leave 41-year-old Dara Torres in the 50 free and Megan Jendrick in the 100 breast as the single American entrants in those events. It was not immediately clear if the U.S. could move a second swimmer already on the team into those events.
Hardy burst onto the international scene at the 2005 world championships in Montreal, where she broke the world record in the 100 breast. Her time of 1 minute, 6.20 seconds still stands as the American record.
She swam at California for two seasons, winning the 100 breast at the 2006 and 2007 NCAA championships before turning pro.