Tyson Gay tested positive for a banned steroid, it has emerged.
American sprinter Gay confirmed last month he had failed a doping test but the substance was not disclosed.
According to extracts of a letter from the United States Anti-Doping Agency (USADA), the 31-year-old's samples showed adverse findings for a banned steroid - which should result in a two-year ban.
The letter, dated July 23, states: "Using the carbon isotope ratio analysis it reported that the sample had an adverse analytical finding reflecting values that are consistent with the administration of a steroid of exogenous (external) nature."
Gay, as well as Jamaican Asafa Powell, tested positive for samples provided at the United States' World Championship trials in June and at an out-of-competition test in May.
Last month Gay said he had been "let down" by someone he trusted.
The sprinter said: "I don't have a sabotage story... I basically put my trust in someone and was let down.
"I hope I am able to run again, but I will take whatever punishment I get like a man."
USADA responded to Gay's disclosure by releasing a statement which read: "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."