North Carolina: "We can confirm that the `B` sample analysis of a sample collected from Mr. Gay has been completed and that the `B` sample analysis has confirmed the `A` sample findings," USADA chief executive Travis Tygart said in a statement to Reuters.
Tygart added an additional sample collected from the American sprinter had also returned "an adverse `A` sample finding", though he did not identify the location of where the second test was taken.
"The results of these tests were expected given Mr. Gay`s acceptance of responsibility for the substances found in his body as soon as he was notified of the `A` sample results," Tygart added.
A tearful Gay said in an interview on July 14 that his `A` sample from the out-of-competition test in May had returned a positive result for a substance he would not disclose.
"I don`t have a sabotage story. I don`t have lies ... I basically put my trust in someone and I was let down," said Gay, who added he had never knowingly taken a performance-enhancing drug.
"I made a mistake," added the 30-year-old, who is jointly the second fastest 100 metres runner of all-time at 9.69 seconds.
Gay`s confirmation of the first positive test occurred on the same day that Jamaican athletics was plunged into crisis.
Former world 100 metres record holder Asafa Powell and Olympic 4x100 metres relay silver medallist Sherone Simpson said they had both tested positive for the stimulant oxilophrine at last month`s Jamaican championships.
Three other athletes failed tests at those championships.
Two-time Olympic 200 metres champion Veronica Campbell-Brown had earlier tested positive for a banned diuretic at a May meeting in Kingston.
Gay, the 2007 world double sprint champion, withdrew from the U.S. team for next month`s world championships in Moscow after notification of May`s positive `A` test.
"We appreciate Mr. Gay voluntarily removing himself from competition prior to the World Championships while we evaluate the circumstances surrounding his adverse analytical findings, and work towards achieving a fair resolution of his case in accordance with the rules," Tygart said.
Gay, the U.S. record holder at 100 metres, sped to what were then the year`s fastest times in winning the 100 and 200 metres at the U.S. championships in late June and had been considered a strong challenger for Jamaican world record holder Usain Bolt at the August 10-18 world championships.
He had recorded the year`s three fastest 100 times, topped by his 9.75 at the U.S. meeting. Only Bolt has run faster than Gay`s 19.74 seconds in the 200 this year.
"It has been tough for me," said the soft-spoken Gay, sobbing throughout the July interview. "I have always been a drug-free athlete."
He had never previously failed a doping test and had participated in a USADA program in which athletes undergo additional testing to prove they are drug-free.
Neither he nor his management team could be reached on Friday.