The Chinese swimmer Chen Xinyi and the Bulgarian steeplechaser Silvia Danekova have tested positive for banned substances at the Rio Olympics.

The Chinese Swimming Association said Chen tested positive for a substance without providing specifics. Xinhua, the official state news agency, reported the story early on Friday.

The 18-year-old finished fourth in the women’s 100m butterfly on Sunday, missing a bronze medal by nine-hundredths of a second. She was also scheduled to compete in the 50m freestyle, which begins on Friday.

There were no immediate details on the positive test, which comes as doping has been a major storyline in the Rio Games.

Seven Russian swimmers have been allowed to compete in Rio after initially being banned following allegations of a huge, state-sponsored doping operation in their homeland. That decision sparked vocal complaints from several swimmers, most notably the American Lilly King whose remarks were aimed at Yulia Efimova, the Russian who on Friday added 200m breaststroke silver to her 100m second place.

Chen’s team-mate Wang Shun declined to comment after he won bronze in Thursday’s 200m medley. “This situation, I don’t know the details. I’m not aware of it, so unfortunately I can’t answer your question,” Wang said.

Li Keke, a spokeswoman for China’s national anti-doping agency, said she had no additional information. “We have noticed media reports about the case. So far the anti-doping agency has yet to receive any official confirmation,” Li said. “This reported test should be generated by the organiser. The Olympic committee and Fina should know about it. At present, we will continue to monitor.”

Fina, the international governing body of swimming, did not immediately respond to a request for comment.

Meanwhile, Danekova has denied taking performance-enhancing drugs and said she was shocked at testing positive for the banned blood booster erythropoietin (EPO) a few days after her arrival in Brazil.

The 33-year-old, who was due to compete in the women’s 3,000m steeplechase on Monday, has reportedly been suspended pending the result of the test on her B sample.

Danekova said the only logical explanation she could give for the positive test was that the substance came from a contaminated food supplement. EPO, which increases the number of red blood cells, has been used mostly by endurance athletes such as middle and long-distance runners and cyclists.

“I’ve been tested four times after entering the Olympic Village,” Danekova, who faces a four-year ban if found guilty, told Bulgarian television on Friday. “[Results] of the three of the samples were negative. It’s an incredibly big shock.”

Danekova, who failed to get beyond the heats in the steeplechase at the 2012 London Olympics, said: “I don’t feel guilty. I cannot tell you how humiliated I feel. I feel robbed emotionally. But we’re coming from the east [eastern Europe], we’re too close to Russia ...”