Atlanta, GA (CNN) — U.S. sprinter Tyson Gay, a former world champion, tested positive for a banned substance and ruled himself out of next month's world championships in Russia.
Gay said Sunday he was told by the U.S. Anti-Doping Agency that an A sample from an out of competition test taken in May came back positive.
He didn't reveal the substance and added that he never knowingly took performance enhancing drugs.
"I don't have a sabotage story," said Gay, 30. "I basically put my trust in someone and was let down. I made a mistake.
"I know exactly what went on, but I can't discuss it right now."
Gay becomes the second track star in a month to be embroiled in a doping scandal.
Two-time Olympic 200-meter champion Veronica Campbell-Brown was provisionally suspended in June after she tested positive for a banned substance.
The Jamaican sprinter reportedly had traces of a banned diuretic, which is used as a masking agent, in a sample she provided to testers at Jamaica's International Invitational World Challenge in May.
British newspaper The Guardian reported the banned diuretic was from a cream she was using in an attempt to recover from a leg injury.
Gay was one of the athletes shown on USADA's website as part of My Victory, "an initiative in the fight to preserve clean sport."
In a statement Sunday, USADA said it "appreciates" Gay's handling of the affair.
"In response to Mr. Gay's statements, USADA appreciates his approach to handling this situation and his choice to voluntarily remove himself from competition while the full facts surrounding his test are evaluated," it said.
"The B sample will be processed shortly, and as in all cases all athletes are innocent unless or until proven otherwise through the established legal process, and any attempt to sensationalize or speculate is a disservice to due process, fair play, and to those who love clean sport."
USA Track and Field CEO Max Siegel said it was "not the news anyone wanted to hear, at any time, about any athlete."
"As we approach the world championships, we will remain focused on the competition at hand and winning the right way," he said in a statement.
Gay posted the fastest time in the 100 meters this year when he clocked 9.75 seconds at last month's U.S. trials in Iowa.
For an athlete who has struggled with injuries, it provided hope -- and a possible challenge to sprint king Usain Bolt -- ahead of the world championships in Moscow that begin August 10.
Gay won gold in the 100 and 200 meters at the 2007 world championships in Osaka but suffered a hamstring injury a month before the 2008 Olympics in Beijing.
He didn't make the 100-meter final.
Last summer at the 2012 Olympics in London, Gay finished fourth in the 100 meters, edged by fellow American Justin Gatlin -- Gatlin once served a four-year ban for doping -- by one-hundredth of a second for the bronze.