(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. 
Did Asafa Powell fail her test?
Asafa Powell tested positive