WASHINGTON, D.C. — Michael Phelps never climbed a starting block in international competition with the confidence he was about to swim against a field free of drug cheats.

“Throughout my career, I have suspected that some athletes were cheating, and in some cases those suspicions were confirmed,” the 28-time Olympic medalist told a congressional subcommittee Tuesday at a hearing on international anti-doping efforts. “Given all the testing I, and so many others, have been through, I have a hard time understanding this. … I can’t adequately describe how frustrating it is to see another athlete break through performance barriers in unrealistic time frames, knowing what I had to go through to do it.”

Phelps’ testimony represented the strongest step he’s taken toward becoming a leading voice in the anti-doping movement. It’s a role he said he wanted to embrace when he retired from competitive swimming after the 2016 Olympics.

Members of the House Subcommittee on Oversight and Investigations expressed astonishment at Phelps’ testimony and the pervasive culture of cheating — particularly the alleged state-sponsored doping program in Russia — he addressed.

“I find it shocking,” said U.S. Rep. Jan Schakowsky, an Illinois Democrat.

Subcommittee members applauded for Phelps after he finished speaking.

The 23-time gold medalist said he was deeply frustrated by the lack of progress he saw in enforcement over his 16-year Olympic career. He called for an urgent move toward independent governance for drug testing.

“Something needs to happen now,” he said. “It’s crushing sports for our youth and everybody else in the world.”

Phelps, dressed in a blue suit, was flanked by shot putter Adam Nelson, who received an Olympic gold medal in 2013, nine years after he competed in Athens. The original gold medalist, Yuriy Bilonog of Ukraine, was stripped for a doping violation detected in a re-analysis of his urine sample.

Nelson described picking up his gold medal in an Atlanta food court, calling it “an afterthought” to the official who handed it over. Cheating prevented him from basking in what should have been his greatest Olympic moment, he said.

“I think we’re at a point now where it needs to be talked about,” he said after the hearing. “So much has happened over the last couple years that this needs to be a major issue that we are trying to talk about and are trying to fix. … I think in my career, for me to be able to do everything I needed to do, I needed to take care of myself mentally, physically, every single way. So for me, I wasn’t going to go and say this or that about this person or that person, because it’s not my business.”

After he finished his career with five gold medals at the Olympics last summer, Phelps said he intended to become more outspoken about doping issues. He was particularly frustrated when athletes who had previously tested positive for banned substances were allowed to swim for Olympic medals.

He did not mince words in Rio when asked about U.S. teammate Lilly King’s criticism of Russian rival Yulia Efimova.

“It’s sad that today in sports in general, not just only swimming, there are people who are testing positive who are allowed back in the sport — and multiple times,” Phelps said at the time. “I believe sport should be clean and sport should be on an even playing field, and I think that it’s sad that in sports toady we have people who are testing positive not only once but twice and still having the opportunity to swim at this Games.”

Subcommittee members focused many of their questions on state-sponsored doping in Russia, a scandal that has loomed over the Olympics in recent years.

The International Olympic Committee missed a major opportunity when it chose not to ban Russia from the Rio Games, said Travis Tygart, chief executive officer for the U.S. Anti-Doping Agency.

Tygart said enforcement will be difficult as long as the IOC and other athletic federations, which are charged with promoting Olympic sports, also have the final say on drug penalties. “It’s the fox guarding the hen house,” he said.

Phelps agreed, saying independent anti-doping enforcement would be crucial to making international competition cleaner.

About 1,900 Olympic athletes were never tested in the run-up to the Rio Games, Tygart said. Phelps, meanwhile, said he was tested 13 times in the six months before the Olympics.

He recounted filling out stacks of paper to let drug testers know where he’d be every day of the year. He said he was generally tested multiple times a month and was sometimes awoken at 6 a.m. on rare days off to submit blood and urine samples.

He never felt confident international rivals were held to the same standard. “Are there people going through the same things I was going through?” he said. “I hope so.”

Phelps, 31, said he never considered using performance-enhancing drugs and remains perplexed that some competitors chose differently. For him, he said, pursuing greatness was about going further in training than he believed his rivals were willing to go.

He told the subcommittee that when kids ask him about pursuing outlandish goals in sports, he wants to reassure them that they can strive on a level field.

“I don’t even know how I would talk to my son about doping in sport,” Phelps said.