Russian curler Aleksandr Krushelnitckii is suspected of failing a doping test at the PyeongChang Olympic Games.
Krushelnitckii, who won bronze in the mixed doubles event at this year's Olympics, is awaiting the results of a so-called "B sample," which could be known as early as this evening local time in South Korea.
OAR team confirm positive doping test, without naming athlete or sport
IOC calls the news "extremely disappointing"
The Court of Arbitration for Sport (CAS) said in a statement Monday it has initiated the process of reviewing Krushelnitckii's case after a request by the International Olympic Committee (IOC). A hearing date has not been set, the statement said.
Earlier in the day, Konstantin Vybornov, a spokesman for the Olympic Athletes from Russia, declined to name Krushelnitckii but said an official comment would be made after the results of the second test.
More than 160 Russian athletes who could prove they were clean from doping are competing in a Olympic Athletes from Russia (OAR) team after the nation was banned from the Games because of state-sponsored doping leading up to and including the 2014 Sochi Winter Games.
Japanese speed skater Kei Saito was sent home from the Olympic village last week after becoming the first athlete at the 2018 Olympic Games to return a positive drugs test. The Court of Arbitration for Sport (CAS) said Saito voluntarily accepted his provisional suspension after an out-of-competition test turned up signs of a masking agent.
In a statement posted to Twitter Sunday, the IOC declined to comment on individual cases, but said it took note of the statement by the OAR spokesman.
The IOC said it "cannot communicate on individual cases while the procedure is still ongoing," because "doping testing and sanctioning at the Olympic Winter Games PyeongChang 2018 is independent from the IOC."
IOC spokesman Mark Adams said at a daily press briefing Monday it would be "extremely disappointing for us if a case is proven."
"On the other hand, what is does show is we have a system which is effective and protects the rights of the clean athletes," he added.
Several media outlets in Russia are reporting the positive test is for meldonium, the same substance for which tennis star Maria Sharapova was banned.
The use of meldonium, which was added to the list of banned substances by the World Anti-Doping Agency from 2016, is widespread in Russia and eastern Europe, where it is often taken as a heart drug.
Meldonium was added to the prohibited list "because of evidence of its use by athletes with the intention of enhancing performance," according to WADA's website.
American curler Matt Hamilton told reporters whatever the circumstances, Krushelnitckii, should be stripped of his medal.
"I feel for the guy if he didn't take it knowingly but you're responsible for what goes into your body so I think they would have to strip him of his medal."
Hamilton said while the benefits of using meldonium isn't immediately apparent in curling, it does have its advantages.
"Curling's a finesse game, but there's a lot of strength and endurance and when you have more strength and endurance, you have a larger window to throw your shot into.
"So there is advantages to it. It's not quite as apparent as something like a cross-country skier or maybe a distance runner or even a weightlifter, but there is absolutely strength and conditioning and fitness in curling and it will give you a little bit of an edge even if it's not on every shot in every end."
Four-time Olympian John Shuster, who's on the same American team as Hamilton, said the doping was unfortunate.
"I've played in the same mixed doubles tournaments as that team and they're good people."
He said it was hard to see stories like this happen.
"You'd never know if it's on purpose or an accident, but that's been banned for a year and a half, and I can't imagine it's something that happened on purpose, so it's just unfortunate."
Hamilton said he felt for the other Russian athletes and how this scandal may affect them.
"I feel mostly bad for Russia because they got banned from this one and if some of their athletes are still getting caught positive, it's sad for them because who knows what the ramifications are going to be.
The news of a doping case in the OAR team could mean the athletes won't be able to march in their national uniform and with the Russian flag at the closing ceremony.
Although they competed under the OAR flag, the IOC had said beforehand the Russian athletes may have been able to march in their own national colors at the closing ceremony if they stuck to its strict code of conduct on neutrality.