It added that a test on 18 February came back positive but a previous one on 13 February had showed a clean result.
The Russian Bobsled Federation president, Alexander Zubkov, told The Associated Press that Sergeeva had a clean test as recently as February 13, and the banned substance was in a sample she provided five days later.
The incident marks the second time a Russian athlete has tested positive during this year's competition and dashes hopes the country would be officially reinstated for the Olympic Games closing ceremony.
Russian Federation was banned from competing at the 2018 winter games in December due to the country's "systemic manipulation" of anti-doping rules.
"This won't win us any extra credit", Russian delegation leader Stanislav Pozdnyakov told members of that country's media, per the AP.
"If we are here, and we are clean", she said, "we should be able to walk under our flag." Erm.
Krushelnitsky and Bryzgalova have agreed to surrender their medals, with the case coming at a delicate time for Russian Federation, which has been accused of running a state-backed, systematic doping program for years, an allegation Moscow denies.
Winter Olympics: Second Russian athlete tests positive for doping
The Court of Arbitration for Sport had been due to hear Krushelnitsky's case Thursday, and did not immediately respond to a request for clarification on whether proceedings would still go ahead. Nadezhda Sergeeva - who has ironically been wearing a shirt reading "I don't do doping" - has become the latest distraction for Russian Federation after the country's flag was banned from the Olympics for cheating.
After finishing 12th in the two-man bobsled on Wednesday, Sergeeva told Yahoo Sports that she was disappointed about competing under the OAR label. She was less than half a second from a podium time.
Two other athletes had doping violations at the Games, a Japanese speedskater and Slovenian hockey player.
There are, on the face of it at least, similarities between the cases involving Krushelnitsky and Sergeeva.
Krushelnitsky tested positive for the banned substance meldonium, which is believed to help blood circulation, after winning bronze in mixed doubles along with his wife Anastasia Bryzgalova.
Sun said he had been prescribed the drug for a medical condition and hadn't known it was banned.
Russia's bobsled program has been in the spotlight for drug use for several years.