(CNN) -- Elite sprinters Tyson Gay and Asafa Powell tested positive for banned substances on a day of shame for athletics.
Gay, a former world champion from the U.S., 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.
Later Sunday, Powell, a former world-record holder from Jamaica, said he was caught for using the banned stimulant oxilofrine that showed up in a test at last month's Jamaican trials.
Jamaica's Sherone Simpson, too, revealed she was caught for doping.
Gay didn't name the substance found in his system and added that he never knowingly took performance-enhancing drugs. He pulled out of next month's world championships in Russia.
"I don't have a sabotage story," Gay was quoted as saying by Reuters. "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 and Powell, both 30, become the second and third high-profile track stars 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 of Jamaica -- 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.
Powell held the world record in the 100 meters for three years prior to Bolt beating it in 2008. He was part of Jamaica's victorious 4x100-meter relay team at the 2008 Olympics.
"I want to be clear in saying to my family, friends, and most of all my fans worldwide that I have never knowingly or willfully taken any supplements or substances that break any rules," Powell said in a statement as reported by the online version of the Jamaica Gleaner. "I am not now, nor have I ever been a cheat.
"This result has left me completely devastated in many respects."
Powell could have been picked to compete in the relay in Moscow but acknowledged that now wasn't a possibility.
Simpson, in a statement also reported by the Gleaner, said she tested positive for oxilofrine.
She won gold at the 2004 Olympics in Athens in the 4x100-meter relay.
"As an athlete, I know I am responsible for whatever goes into my body," the 28-year-old said. "I would not intentionally take an illegal substance of any form into my system."