Alexander Krushelnitsky still has to be officially named and undergo a second test, but if confirmed the incident could keep the Russia's banned team from being reinstated and marching under the national flag at the closing ceremony.
Russian curlers said on Monday a coach told them that mixed doubles bronze medalist Alexander Krushelnitsky tested positive for a banned substance at the Pyeongchang Olympics.
Women's skip Viktoria Moiseeva said the coach "came to tell us the news" late on Sunday.
She added the team wanted to comfort Krushelnitsky and his wife and curling partner Anastasia Bryzgalova but "we thought that there are no words to comfort now. We just tried to stay away."
"I would never believe someone on our team would do that," she said.
"I don't know how he could do such a thing and sleep at night because it's not only him, but it's the whole nation."
Russian Curling Federation president Dmitry Svishchev would not confirm the name of the athlete who tested positive, but told The Associated Press that Krushelnitsky tested clean as recently as January 22, the day before he flew to a pre-Olympic training camp in Japan.
Svishchev said it was possible that an athlete's food or drink had been spiked with meldonium, which was banned in 2016, and suggested rival Russian athletes or Russia's political enemies could be responsible.
"It can't happen at the Olympic Village because everyone eats the same canteen food," he said.
"It could happen at training camp or in the intervening period ... There's a possibility of it being something within the team, that something happened during training camp, or as a political means to achieve some goal."
The curling team trained in Japan in January, bringing in some Russian athletes who had not qualified for the Olympics as training partners.
Meldonium is the substance that tennis star Maria Sharapova was suspended for after testing positive at the Australian Open in 2016. Before it was banned, many Russian athletes used the drug, which is designed for people with heart problems.
IOC spokesman Mark Adams says a failed doping test by a Russian athlete could keep the country's banned team from being reinstated and marching under the national flag at the closing ceremony.
Adams confirmed the positive test and says it could have "consequences" in evaluating the behavior of Russian athletes. He declined to name the athlete.
A positive sample
The athlete's "A sample" tested positive. Adams said results of a second sample would be tested, and results could be announced within 24 hours.
The Norwegian team finished fourth, and could get the bronze if the positive test is confirmed.
Russian athletes are participating in the Pyeongchang Olympics as "Olympic Athletes from Russia."
The IOC suspended the Russian Olympic committee last year in connection with a massive doping scheme at the 2014 Olympics in Sochi but allowed 168 athletes to compete under neutral uniforms and without the Russian national flag.
Adams says Russians at the games have undergone "rigorous testing" and adds that "Russians were tested to a significant level more than others."
The case was the talk of the rink at curling.
Though to the uninitiated, the idea of a curler using performance-enhancing drugs may seem bizarre, the sport does demand a high level of athleticism at the Olympic level.
Curlers need to have strong core muscles and upper body strength in order to manage the often rigorous sweeping that helps them guide the rock down the ice.
Fitness is even more important in mixed doubles, the event at which Krushelnitsky was competing. Because there are just two curlers on each mixed doubles team instead of the four in traditional curling, there is little rest between throws, and both teammates are often heavily involved in sweeping.
"It's physically demanding out there. It's not like you don't need any muscles," said Swiss curler Silvana Tirinzoni, whose team beat the Russian women's team 11-2 at Monday's round robin.
"We have to be fit and we have to be working out. My sweepers are working out like five times a week, go to the gym, so it (performance-enhancing drugs) can help, but we shouldn't do that. I think that's stupid."
Tirinzoni said if the reports were confirmed, they would be disappointing. But she said she still had no reservations about playing against the Russian athletes on Monday.
"I think clean athletes have the right to compete," she said. "It's not about me to make those kind of decisions. So I believe the team we played today, they are as clean as we are."