Relay sprinter Debbie Dunn took her name off the U.S. Olympic team roster Friday after testing positive for excessive testosterone.

Dunn, who finished fourth in the 400 meters at Olympic trials, was selected for the American relay pool. She is the 2010 world indoor champion at 400 meters and would have been a likely candidate to run in the Olympic 1,600-meter relay, which the American women have won every year since 1996.

But she released a statement acknowledging a positive doping test and said she was withdrawing from the Olympics while the U.S. Anti-Doping Agency pursues the case.

"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. "As in all cases, all athletes are innocent until and unless proven otherwise through the established full, fair legal process which was approved by athletes, the U.S. Olympic Committee, and all Olympic sports organizations."

No replacement was immediately named, and it may not be necessary to find another sprinter to take Dunn's place. Among the candidates to run in the 1,600-meter relay are the top three finishers in the 400 at trials — Sanya Richards-Ross, Francena McCorory and DeeDee Trotter, along with Diamond Dixon, Keshia Baker and Allyson Felix, who qualified for the 100 and 200 sprints but also has experience running at 400 meters.

USA Track and Field CEO Max Siegel said Dunn did the appropriate thing by withdrawing.

"At this point, her case is being handled by the U.S. Anti-Doping Agency, which will adjudicate the matter," Siegel said.