Disgraced U.S. cyclist Floyd Landis revealed new cheating allegations in a series of messages to sponsors and officials, alleging that former teammate Lance Armstrong not only joined him in doping but taught others how to beat the system and paid an official to keep a failed test quiet.

With international cycling season in full swing, 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 de France title.

His fresh accusations, contained in e-mails sent last month, prompted Armstrong to hold an impromptu press conference Thursday at the Tour of California.

"If you said, 'Give me one word to sum this all up,' credibility," the seven-time Tour de France winner said. "Floyd lost his credibility a long time ago."

"We have nothing to hide. We have nothing to run from," said Armstrong, who later quit the race to go to a hospital for X-rays after crashing just outside of Visalia, Calif. Team spokesman Philippe Maertens told The Associated Press that Armstrong got stitches in the left elbow and under the left eye.

Though Landis lost his title, he denied cheating until now, and his recent e-mails detail his blood doping.

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

He claims that Armstrong and longtime coach Johan Bruyneel paid an International Cycling Union official to cover-up a test in 2002 after Armstrong purportedly tested positive for the blood-boosting drug EPO. The UCI, however, denied changing or concealing a positive test result.

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, maybe just as confused as you guys," Armstrong said, with Bruyneel by his side. "The timeline is off, year by year."

The Wall Street Journal first reported the details of the e-mails.

Landis also implicated other cyclists, including longtime Armstrong confidant George Hincapie and Olympic medalist Levi Leipheimer, and acknowledged using human growth hormone starting in 2003. The Wall Street Journal reported another e-mail from Landis also linked another top American racer, Dave Zabriskie, to doping.

"Look forward to much more detail as soon as you can demonstrate that you can be trusted to do the right thing," Landis wrote in the e-mail to Johnson.

Landis said he was asked at one point to stay in an apartment where Armstrong was living and check the temperature in a refrigerator where blood was being stored for future transfusions. "Mr. Armstrong was planning on being gone for a few weeks to train he asked me to stay in his place and make sure the electricity didn't turn off or something go wrong with the refrigerator," Landis wrote.

Hincapie said he was "really disappointed" by the allegations. Jim Ochowicz, a former top USA Cycling official — who was also implicated by Landis — defended himself and Hincapie.

"These allegations are not true, absolutely unfounded and unproven," said Ochowicz, now the president of BMC Racing, Hincapie's current team. "This is disappointing to anyone who works in the sport or is a fan of the sport."

Johnson said 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, and Landis did not respond to messages left by The AP.

More accusations from Landis could be coming, however. In his e-mail to Johnson, Landis indicated he has several diaries detailing other experiences.

"I've always known Floyd as an angry person ... somebody who's basically angry with the world," Bruyneel said. "To me it sounds like he just wants to drag down people who are still there and enjoying this."

Until about 2005, Armstrong worked extensively with Michele Ferrari, an Italian doctor who was linked to numerous doping issues, but was cleared by an appeals court in 2006. Landis claimed Ferrari extracted "half a liter of blood" from him in 2002, so he could have it transfused during the Tour de France.

"Mr. Armstrong was not witness to the extraction but he and I had lengthy discussions about it on our training rides during which time he also explained to me the evolution of EPO testing and how transfusions were now necessary due to the inconvenience of the new test," Landis wrote.

Andy Rihs, the owner of the Phonak team for which Landis rode when he won the Tour, issued a statement saying Landis' claims were "lies" and a "last, tragic attempt" to get publicity. In the April 30 e-mail, Landis alleges that Rihs was aware of his doping and helped fund it.

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

"He already made those accusations in the past," McQuaid said. "Armstrong has been accused many times in the past but nothing has been proved against him. And in this case, I have to question the guy's credibility. There is no proof of what he says. We are speaking about a guy who has been condemned for doping before a court."

Armstrong said Landis started threatening him and other top riders such as Leipheimer and Zabriskie to make allegations like these long ago.

"I'd remind everybody that this is a man that's been under oath several times and had a very different version," Armstrong said. "This is a man that wrote a book for profit that had a completely different version. This is somebody that took, some would say, close to $1 million from innocent people for his defense under a different premise. Now when it's all run out the story changes."