Floyd Landis admitted to doping, says Armstrong was involved.

The leaders of the IOC and World Anti-Doping Agency said on Friday that Floyd Landis should provide concrete evidence to back up his allegations of doping by seven-time Tour de France champion Lance Armstrong.

"He has to bring proof that this is true," International Olympic Committee president Jacques Rogge told AP. "These are accusations that need to be corroborated by proof.

"You can't condemn without proof," Rogge added. "He would be better off by giving evidence to corroborate that, otherwise he is risking a lot of libels .... You can only sanction an athlete with tangible proof."

WADA president John Fahey, in a separate interview with the AP, said if there is any substance to Landis' allegations, either the U.S. Anti-Doping Agency or International Cycling Union should intervene.

"If he has evidence, he should make that evidence available to the USADA or UCI and I'm sure if there is any substance to that evidence, either of those bodies would act," Fahey said. "There will always be rumors about it."

Former UCI president Hein Verbruggen, meanwhile, denied Landis' allegation that he helped cover up a positive drug test by Armstrong in 2002.

"He has never been (tested) positive," Verbruggen told the AP.

The international officials spoke after Landis, in a series of e-mails sent to sponsors and sports officials, confessed to years of doping after having previously denied cheating.

The American rider, who was stripped of the 2006 Tour de France title and served a two-year ban for doping, also alleged that Armstrong not only joined him in doping but taught others how to beat the system.

Armstrong has denied the claims by his former teammate, saying Landis has no credibility.

"We have nothing to hide," Armstrong said at an impromptu news conference before the fifth stage of the Tour of California. "Credibility, Floyd lost his credibility a long time ago."

Rogge said UCI officials will require "more evidence than just an e-mail. They need to have more details to launch an inquiry."

Rogge also expressed doubts about Landis' claim that Armstrong and longtime coach Johan Bruyneel paid Verbruggen to cover up a test in 2002 after Armstrong purportedly tested positive for the blood-boosting drug EPO.

"To my knowledge it is not possible to hide a positive result," Rogge said, adding that each doping sample has a code known to laboratory testing teams. "The lab knows the code. WADA gets it also. Then it goes to the national and international federations.

"One person cannot decide: 'I can put this under the carpet.'"

Verbruggen said there was never any positive test in the first place.

"Everyone can have a lot of doubts and say whatever they want - the guy has never been positive," the Dutch official said. "Never has Lance Armstrong been declared positive by a lab."

Verbruggen said Armstrong made one visit to UCI headquarters at Aigle, Switzerland, in 2002 after the center's new indoor training track had opened.

"It was a Monday morning," he said. "I remember that because we had a lot of the pupils and youngsters there. It was a great thing for them to be in the picture with him."

Verbruggen said he was told a few weeks ago that Landis would be making his allegations.

"The guy has been lying for three years and spending zillions of money to defend his own lies," Verbruggen said. "Now he is broke and he comes out with a different story."

The UCI issued a statement denying changing or concealing a positive test result, and Bruyneel said, "I absolutely deny everything (Landis) said."

Rogge welcomed Landis's confession of his own doping.

"The fact that he is coming out is something that we applaud," he said. "It will clear his conscience. An admission is proof under the WADA Code and you should be penalized."

Fahey, reached by phone in Melbourne, Australia, said Landis' confessions didn't surprise him.

"There was absolutely no doubt about the decision in the Court of Arbitration for Sport on his final appeal," Fahey said. "They saw him as being a cheat, and in this context, he has now admitted it, and I am pleased. There is no contrition, however, no apology, and I regret that."

In two e-mails obtained Thursday by The Associated Press, Landis admitted for the first time what had long been suspected - that he was guilty of doping for several years before being stripped of his 2006 Tour title.

"I want to clear my conscience," Landis told ESPN.com. "I don't want to be part of the problem any more."

Neither Landis nor his family returned repeated messages from the AP.

The Wall Street Journal first reported the details of the e-mails. The newspaper also reported Landis was cooperating with the Food & Drug Administration's criminal investigations unit and had met with FDA special agent Jeff Novitzky, the lead investigator in the BALCO case.

In an e-mail Landis sent to USA Cycling chief Steve Johnson, he said Armstrong's positive EPO test was in 2002, around the time he won the Tour de Suisse. Armstrong won the Tour de Suisse in 2001 and did not compete in 2002.

"We're a little confused," Armstrong said.

The e-mail to Johnson also said: "Look forward to much more detail as soon as you can demonstrate that you can be trusted to do the right thing."

Landis also implicated at least 16 other people in various doping acts, including longtime Armstrong confidant George Hincapie, Olympic medalist Levi Leipheimer and Canadian cyclist Michael Barry.

The Wall Street Journal reported another e-mail from Landis also linked another top American racer, Dave Zabriskie, to doping.

"At the end of the day, he pointed the finger at everybody still involved in cycling," Armstrong said.

Landis is part of a long list of former Armstrong teammates and former U.S. Postal Service riders who have either acknowledged or been caught doping.

USA Cycling would not comment about Landis' series of e-mails, citing its policy on not discussing "doping allegations, investigations or any aspect of an adjudication process." The U.S. Anti-Doping Agency also declined comment for similar reasons.

Like Armstrong, UCI president Pat McQuaid questioned Landis' credibility.

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