"I do not want any issue like this to distract from my teammates' focus for the biggest meet of their lives," Dunn said. "I wish Team USA [the] best in London as I work toward resolving this matter."
The Chicago Tribune first reported Dunn's positive test.
USADA CEO Travis Tygart said the agency is analyzing Dunn's 'B' sample. If that comes back positive, Dunn would have the choice of accepting a sanction or taking the case to arbitration.
"USADA appreciates Ms. Dunn voluntarily removing herself from the Olympic team while the full facts … are evaluated," Tygart said in a statement.
India's tennis team in disarray
Indian tennis has turned into a soap opera . The conflict centers on Leander Paes, India's most recognizable face in tennis, but an athlete who has apparently burned bridges with nearly every potential teammate.
Paes won a bronze medal in singles at the 1996 Atlanta Games, breaking a 44-year drought of individual Olympic medals for India. At 39, he's still considered one of the country's best medal hopes as one of the world's top doubles players. The problem is there's a lack of players willing to partner with him.
He was supposed to play with former partner Mahesh Bhupathi, with whom he won 26 doubles titles on the professional tour.
But Bhupathi, ranked 14th in the world in doubles, refused to play with Paes because he had been teamed with Rohan Bopanna on the ATP circuit in preparation for the Olympics. Bopanna, ranked 12th, turned Paes down for the same reason.
Indian tennis officials scrambled to partner Paes with 307th-ranked Vishnu Vardhan, whom Paes proceeded to publicly belittle.
"I don't think he's been to Wimbledon before," said Paes, ranked seventh in doubles. "I don't even know if he has grass-court shoes, poor guy."
Indian tennis officials had to sweeten the deal, promising Paes could play mixed doubles, considered a more realistic medal opportunity, with India's top female player, Sania Mirza, who had just won the French Open mixed doubles with Bhupathi. Mirza was not happy.
"As an Indian woman belonging to the 21st century, what I find disillusioning is the humiliating manner in which I was put up as a bait to try and pacify one of the disgruntled stalwarts of Indian tennis," she said.