Revel - Greg LeMond believes a federal probe into fellow Tour de Frace winner Lance Armstrong should not be taken lightly, and could even bring about the downfall of the world's most famous cyclist.
"Up until now, he has achieved great things, if you consider he did it fairly, which I don't believe," LeMond said in an interview conducted in French with the Journal Du Dimanche newspaper.
"For him, it's the beginning of the end."
Seven-time champion Armstrong is racing his final Tour campaign amid damaging accusations by former teammate Floyd Landis that their former team, US Postal, was involved in systematic doping practices.
A federal investigation into Landis's claims has been launched and is being led by Jeff Novitzky, the same federal agent whose probe into the BALCO doping scandal brought about the downfall of athletics star Marion Jones.
Grand jury subpoenas were issued to potential witnesses in the probe this week in a move that demonstrates how seriously the authorities are taking allegations made by Landis.
LeMond, along with several of Armstrong's former teammates, is one of several witnesses who has been issued a subpoena. Although he has "yet to decide" on whether he will go, his testimony could be valuable.
The three-time yellow jersey champion has been an advocate of clean cycling for the past decade, and subsequent questioning of Armstrong's record-setting performances have led to the pair having a turbulent relationship.
Some fans claim LeMond is bitter because Armstrong went on to surpass his record for an American on the race, but he has not limited his queries to Armstrong. Last year he questioned whether Spain's reigning champion, Alberto Contador, was riding clean.
LeMond said he has taken no particular enjoyment from seeing Armstrong suffer on what has been a disastrous farewell campaign.
He believes the Texan faces more pressing concerns than the multiple crashes he has suffered in the past two weeks.
"Seeing him suffer doesn't affect me at all. I would even have preferred it if he hadn't crashed," added LeMond, who won the race in 1986, 1989 and 1990.
He added: "The federal investigation is very serious, more than people believe.
"Given everything that he has been accused of recently, I'm even surprised he decided to race the Tour. I don't know how he's managed to stay concentrated on the race.
"It will be interesting to see if he collaborates with the investigation."
Armstrong said earlier this week he would be prepared to cooperate with any investigation, provided it did not become a "witch hunt".
"Like I said, as long as we have a legitimate and credible and fair investigation, we'll be happy to cooperate, but I'm not going to participate in any kind of witch hunt," he said.
And while Armstrong continues to question the credibility of Landis, who denied for four years that he had doped before finally confessing two months ago in a bid to "clear his conscience", LeMond has no doubts.
He claimed that friends of Landis were even being threatened by Armstrong.
"Listen, Landis spoke out because Armstrong was going after him. He made threats against his (Landis's) friends," added LeMond.
"I believe Landis because everything he's said, I've already heard. There's a major difference between a guy like Ivan Basso (who was banned for two years for doping) and Armstrong. Basso doesn't threaten people!
"When it comes to manipulating people, Armstrong is the undisputed champion."