WASHINGTON (AP) — Testifying before a congressional hearing on improving anti-doping measures, Michael Phelps has expressed frustration over seeing others cheat and says athletes need to believe the system works.

The retired swimmer said Tuesday he has a hard time understanding how athletes get around anti-doping tests and that when they do, it's "disillusioning." Phelps, the most decorated Olympian, with 28 medals, asked the House Energy and Commerce Subcommittee on Oversight and Investigations to help "ensure the system is fair and reliable."

Phelps said he doesn't believe he has always been in competitions in which the entire field was clean.

Shot putter Adam Nelson, who received a gold medal after the original winner at the 2004 Athens Olympics tested positive for doping, told his story and asked Congress to "give meaning to my medal." Nelson picked up his gold medal at the food court at the Atlanta airport but said it didn't mean the same.