This week, Michael Phelps appeared before a Congressional subcommittee to testify about the issue of doping, but a rival from Phelps’ past came forward this week with questions about why Phelps is tackling the issue post-retirement.

Serbian world champ Milorad Cavic openly questioned Phelps’ anti-doping stance in an open letter of sorts this week, posted on Twitter:

Cavic criticizes Phelps for speaking out for doping reform only after his retirement, suggesting Phelps hadn’t supported blood passports when he was still convenient. Cavic (also retired) suggested the timing of Phelps’ stand was “convenient,” and though he says he’s not suggesting Phelps is a cheat, he categorizes Phelps’ recovery rate as “nothing short of science fiction.”

The reference to blood passports may stem from a program Cavic tried to popularize back in 2009 where athletes would undergo “athlete biological passports” which track biological markers of doping and results of anti-doping tests over a series of years. Cavic wanted athletes (Phelps included) to make their results fully public on his website, according to an ESPN story from the time.

You can see Phelps’ testimony before the subcommittee here.

Phelps and Cavic went head to head in several Olympics, most notably the 2008 Beijing Games. Cavic very nearly ended Phelps’ dream of 8 gold medals at those games, pushing the 100 fly down to a touchout where Phelps won by just .01 seconds in one of the most dramatic finishes all-time.

Cavic does conclude his letter by expressing his hope that Phelps will continue to champion anti-doping causes to make a difference in the sport.

A full transcript of Cavic’s statement:

Dear Michael,

Doping has been a problem and it’s only getting worse. I, too, don’t know what to tell my son, nor would I wish that my son ever be half as good as I was knowing what he’ll face tomorrow. People get tested, some more than others… I could even recall Lance Armstrong getting tested 3x in one day and never failed once, but that’s not the problem. At the moment, we’re not able to detect new drugs and advanced methods of doping. Why you’re seeking reform now that you’re retired, and never before supported blood passports, is beyond us all, perhaps even convenient. I’m not suggesting you’re a cheat, you’ve gradually improved your times throughout your career, but your recovery rate is nothing short of science fiction… We all just wished we could understand it. Anyway, I really do hope that you’ll stick with this, because incase our sons go pro some day, I’d like to think you made a difference #NeverTooLate