Powell, 30, was found to have a stimulant called oxilofrine in his system and his fellow Jamaican Sherone Simpson also tested positive for the same stimulant.
Powell held the 100-metre record of 9.74 until Usain Bolt beat it in 2008. Simpson won Olympic gold in the women's 400m relay in 2004 and silver in 2012, along with an individual silver in the 100m in 2008.
Both athletes denied knowingly taking the banned substance.
In a statement released through his agent, Powell said, "I assure you that we will find out how this substance passed our rigorous internal checks and balances and design systems to make sure it never happens again."
Gay, who won the 100m and 200m at US nationals last month, failed an out-of-competition test May 16.
The 30-year-old didn't reveal what the banned substance was. He would have his "B" sample tested for a further step.
"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.
"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," announced USADA in a statement.
Gay, who won three gold medals at 2007 world championships, had signatured himself as a man of "clean competition". He joined USADA "My Victory" program, which encouraged clean competition.
"My career and my name have always been better than medals or records or anything like that. I've always wanted a clean name with anything. Unfortunately, I have to break this news, that I have a positive 'A' sample," said Gay.