(Reuters) - Michael Phelps has rejoined the U.S. Anti-Doping Agency's drug testing program in a signal the most decorated Olympian could be considering a return to the swimming pool in time for the Rio de Janeiro Games in 2016.
Phelps retired after winning four gold and two silver medals at the 2012 London Olympics to take his career tally to 22, 18 of which were gold.
The 28-year-old was tested twice in the quarter ended September 30, according to data released by USADA. World swimming governing body FINA requires athletes to be tested for a minimum of nine months before competing in sanctioned events.
Phelps had repeatedly said before the London Games he would not contemplate a comeback after he retired and his coach Bob Bowman downplayed any suggestions he was considering a return to compete at what would be his fifth Olympics in Rio.
"It's premature to say that," Bowman told USA TODAY Sports on Thursday. "What we're doing is kind of letting him have his options.
"He came back this fall and started to do some training with the group, mainly just to get in shape. He just felt like he was not fit. He wasn't.
"He's occasionally been training. He's picked it up a little more. We were just thinking about it, and I said, 'You know, you're getting in pretty good shape, maybe you want to swim in a meet?' He said, 'Well, maybe at some point.'"
Writing by Sudipto Ganguly; Editing by Greg Stutchbury