American sprinter Tyson Gay, the fastest man over 100 metres this year, has tested positive for a banned substance and said he would pull out of the world championships next month in Moscow. Gay wouldn't reveal the substance in a phone conversation from his training base in Amsterdam, but he said he was notified by the US Anti-Doping Agency late last week that a sample came back positive from a May 16 out-of-competition test. He said he would have his "B" sample tested soon, possibly as early as this week. "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," said Gay, who fought back sobs as he spoke. "I don't have any of those stories. I basically put my trust in someone and I was let down." Asked who that person was, Gay replied: "I can't really say it. Sometimes a human being naturally, generally trusts somebody. That's what people do." A triple world champion in 2007, Gay was healthy again this season after being constantly plagued by hamstring and groin ailments, along with a surgically repaired hip. A few years ago, Gay was part of Usada's programme called "My Victory", where athletes pledge to compete clean. In his testimonial on the website, Gay said: "I compete clean because I really believe in fairness, and besides that, my mom would kill me! Just being honest." He's spoken with his teammates, friends and family. "They already know it is some type of accident, or some type of - I don't want to use certain words, to make it seem like an accident, because I know exactly what went on, but I can't discuss it right now," he said. "I've always wanted a clean name with anything. Unfortunately, I have to break this news, that I have a positive 'A' sample."