Asked repeatedly what might have tripped his test, Landis refused to lay blame on any one thing. "As to what actually caused it on that particular day, I can only speculate," he said.

Landis had an exemption from the Tour to take cortisone shots for pain in his hip, which will require surgery for a degenerative condition, and was taking an oral medication for hyperthyroidism. He and his doctor were consulting with experts to see if those drugs might have thrown off his testosterone levels.

Landis said he was still in Europe, but declined to say exactly where. "Not to be elusive, I have to figure out a way to get to the airport and get home," he said.

Arlene Landis said her son called Thursday from Europe and told her he had not done anything wrong.

"Lance (Armstrong) went through this too," she said in an interview with the AP at her home in Farmersville, Pa. "Somebody doesn't want him to win.

"Why do they put you through two weeks of misery and spoil your crown? My opinion is when he comes on top of this everyone will think so much more of him. So that's what valleys are for, right?"

USA Cycling spokesman Andy Lee said the organization could not comment until the process is complete. Carla O'Connell, publications and communications director for the US Anti-Doping Agency, also had no comment.

UCI spokesman Enrico Carpani said Landis requested the "B" sample, which is not required by cycling's governing body.

"We are confident in the first (test)," Carpani said. "For us, the first one is already good."

Tour director Christian Prudhomme stressed that the backup test still must be done, and it would be up to the UCI to determine penalties. "It is obviously distressing," he said at a Paris news conference.

Under World Anti-Doping Agency regulations, a ratio of testosterone to epitestosterone greater than 4:1 is considered a positive result and subject to investigation. The threshold was recently lowered from 6:1. The most likely natural ratio of testosterone to epitestosterone in humans is 1:1.

Testosterone is included as an anabolic steroid on WADA's list of banned substances, and its use can be punished by a two-year ban.

Testosterone can build muscle and improve recovery time when used over a period of several weeks, said Dr. Gary Wadler, a member of the World Anti-Doping Agency and a spokesman for the American College of Sports Medicine. But if Landis had been a user, his earlier urine tests during the tour would have been affected, he said.

"So something's missing here," Wadler said. "It just doesn't add up."

Landis' inspiring Tour was reminiscent of fellow American Tyler Hamilton's gutsy 2003 ride. Hamilton, riding for team CSC, broke his collarbone on the first day of the Tour but rode on, despite the pain, and finished fourth overall.

But a year later, Hamilton, then riding for Phonak, tested positive for blood doping at a Spanish race and now is serving a two-year ban. He has denied blood doping.

Speculation that Landis had tested positive spread earlier Thursday after he failed to show up for a one-day race in Denmark on Thursday. A day earlier, he missed a scheduled event in the Netherlands.

On the eve of the Tour's start, nine riders - including pre-race favorites Jan Ullrich and Ivan Basso - were ousted, implicated in a Spanish doping investigation.

The names of Ullrich and Basso turned up on a list of 56 cyclists who allegedly had contact with Spanish doctor Eufemiano Fuentes, who's at the center of the Spanish doping probe. Landis was not implicated in that investigation.