LONDON (AP) -- The credibility of track and field's anti-doping program has been "enhanced, not diminished" after sprinters Tyson Gay and Asafa Powell tested positive for banned substances, the IAAF said Monday.
Gay, a former world champion who won the 100- and 200-meters at U.S. nationals last month, said he would pull out of next month's world championships in Moscow.
Powell, who held the world record in the 100 until Usain Bolt lowered it in 2008, and Olympic gold medalist Sherone Simpson also face suspension after failing tests at the Jamaican championships last month.
IAAF spokesman Nick Davies said the sport's governing body does not comment on pending cases, but added that the fight against doping "is enhanced, not diminished, each time we are able to uncover a new case."
"The IAAF's commitment to anti-doping in athletics is unwavering because we have an ethical obligation to the majority of athletes who believe in clean sport," Davies said in a statement. "The fact that we are able to detect and remove from the sport athletes who have breached our anti-doping rules should be seen in this context."
The sport was thrown into turmoil on Sunday when news emerged of the failed tests. The 30-year-old Powell called for an investigation into how a stimulant called oxilofrine entered his system.
"I am not now - nor have I ever been - a cheat," Powell posted on Twitter.
Simpson, who tested positive for the same stimulant, said she "would not intentionally take an illegal substance of any form into my system."
The positives recorded by Powell and Simpson are part of a bigger doping crisis hitting Jamaica, the home of Bolt and a country which has dominated sprinting medals at recent Olympics.
In Sunday's editions, Jamaican newspaper The Gleaner reported that five athletes had tested positive. Paul Doyle, the agent who represents Powell and Simpson, confirmed to The Associated Press that his sprinters were among them.
Shortly after Doyle's confirmation, Powell and Simpson each released statements acknowledging the positive tests.
The news came a month after another Jamaican Olympic gold medalist, Veronica Campbell-Brown, tested positive for a banned diuretic.
Campbell-Brown is being suspended while a disciplinary panel reviews her case. She has denied cheating.
Gay, who won world championships in the 100, 200 and 4x100 relay in 2007, previously took part in the U.S. Anti-Doping Agency's "My Victory" program - in which athletes volunteer for enhanced testing to prove they're clean - and his results never raised red flags until an out-of-competition test on May 16.
"I don't have a sabotage story. I don't have any lies. I don't have anything to say to make this seem like it was a mistake or it was on USADA's hands, someone playing games," Gay said in a telephone interview. "I don't have any of those stories. I basically put my trust in someone and I was let down."
Neither Gay nor the USADA revealed the banned substance at the center of the positive sample.
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