FILE - In this March 2007, file photo, U.S. swimming star Michael Phelps, left, walks with his coach Bob Bowman during a training session at the World Swimming Championships in Melbourne, Australia. Phelps is back in the U.S. drug-testing program, the strongest sign yet that he's returning for the 2016 Rio Olympics. The U.S. Anti-Doping Agency says Phelps was among the athletes who underwent doping tests in the third quarter, the period ending Sept. 30. He was tested twice. (AP Photo/Mark Baker, File)Michael Phelps has rejoined the U.S. drug-testing application, the strongest sign but that he is returning for the 2016 Rio Olympics.