Fifteen of the 558 athletes in the U.S. delegation at the Rio Olympics - or slightly less than 3 percent - had therapeutic-use exemptions in force during the games.

The exemptions, known as TUEs, came under increased scrutiny last week after Russian hackers broke into the database of the World Anti-Doping Agency and posted confidential medical information online from some athletes.

TUEs let athletes use otherwise-banned substances to treat long-standing medical conditions, such as attention deficit disorder and asthma. Proponents argue those exemptions only level the playing field; critics say they can give competitors an edge.

The hackers largely targeted female U.S. Olympians, among them tennis stars Serena and Venus Williams and gold medal-winning gymnast Simone Biles. All three, as well as several other prominent athletes whose medical records were posted, said the data strongly supported their use of a TUE.

"I am one of the strongest supporters of maintaining the highest level of integrity in competitive sport and I have been highly disciplined in following the guidelines," Venus Williams said in a statement.

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How the U.S. numbers compare with that of other Olympic delegations is difficult to say - let alone how they compare with the number of TUEs granted by the NFL, NBA, NHL and MLB because of differing standards and reporting practices.

Olympic athletes can request - and receive - a TUE from either their international sports federation or a national anti-doping agency. All exemptions are forwarded to WADA, which does not grant TUEs but can appeal them, and then provided to the International Olympic Committee ahead of the games.

Both WADA and the IOC declined several requests from The Associated Press for those numbers before, during and after the Olympics.

The U.S. delegation total was provided by the U.S. Anti-Doping Agency. According to the agency's 2015 annual report, 136 of the 2,500 or so athletes in its elite Olympic programs - about 5 percent - were granted TUEs in 2015.

Travis Tygart, CEO of USADA, maintains there's little abuse of TUEs in sports that use WADA-accredited protocols. In addition to the documentation required, he said the doctors and scientists who review requests can distinguish between legitimate medical needs to treat chronic conditions (i.e., corticosteroids like prednisone for asthma) from performance-enhancers (often anabolic and androgenic steroids).

"The truth is, it would be foolish to try and cheat this way. If you're trying to cheat, the idea that you would self-disclose what you're already using, or want to use that is illegal, isn't very bright," Tygart said. "Most of the action by cheats now is focused on substances we haven't yet found, or had a chance to ban - like the meldonium situation ."

Among the U.S. professional sports leagues, MLB adheres closest to WADA protocol, both in TUE application and review process. Unlike WADA, however, baseball also reports the number issued each year - 113 for the nearly 1,350 players who appeared in games during the 2015 season.

But that transparency has come with a cost: 111 of those TUEs were approved for Adderall, an amphetamine-based drug prescribed to treat ADD; critics note the number of baseball players being treated for ADD is more than double that of the U.S. adult population as a whole.

The NFL, NBA and NHL declined a request from the AP seeking the number of TUEs issued during the previous season. All three leagues devised their own application and review processes incorporating some aspects - but not all - of WADA's protocol, known as the International Standard for Therapeutic Use Exemptions, or ISTUE.