WARSAW, Poland — Polish cross-country skier Kornelia Marek tested positive for EPO at the Vancouver Olympics in the first serious doping case of the games.

Marek tested positive after helping Poland to a sixth-place finish in the women's 20K relay on Feb. 25, the Polish Olympic Committee said Thursday.

Marek also was a member of the Polish team that finished ninth in the team sprint. She placed 11th in the 30K mass start, 39th in the 10K freestyle and 35th in the 15K pursuit.

If found guilty of doping by the International Olympic Committee, Marek and the relay teams would be disqualified and stripped of their Vancouver results. Marek would also be banned from the 2014 Winter Olympics in Sochi, Russia.

Marek denied taking any banned substances and is awaiting the testing of her backup "B'' sample on Friday at the Vancouver doping lab.

"I hope it will be negative," Marek said on Polish television. "I have nothing to reproach myself for. I was never taking any banned substances."

The Vancouver Games, which ended Feb. 28, had produced only two minor doping violations — both involving hockey players who tested positive for light stimulants and were let off with reprimands.

EPO is a synthetic hormone which enhances endurance by boosting the production of oxygen-rich red blood cells in the body.

The IOC said it was investigating a positive test, but declined to name the athlete because the case hadn't been finalized.

"We can confirm that we have an adverse analytical finding," IOC spokesman Mark Adams said. "The athlete and the National Olympic Committee have been informed and a disciplinary procedure is underway."

The Polish Olympic Committee said it was notified of the positive result by the IOC late Monday. The results of the "B'' sample test will be known Tuesday or Wednesday, spokesman Henryk Urbas said.

"Then we will know if we are dealing with a case of doping," Urbas told The Associated Press. "She is shocked, the coach is shocked, everyone is shocked."

Marek is skipping a cross-country World Cup event in Norway this week to appear before national ski authorities in Poland.

"This is a powerful blow to the Polish Olympic Committee, because we had a very good performance (in Vancouver), the best at a Winter Games so far, and this tarnishes the general picture," Urbas said.

If found guilty, Marek would become the first athlete disqualified from the Vancouver Games for doping. She could also face a two-year ban from the international ski federation. And, under IOC rules, any athlete who receives a doping ban of at least six months is ineligible to compete in the next Olympics.

During the Vancouver Games, the IOC issued reprimands to Russian female hockey player Svetlana Terenteva and Slovakian hockey player Lubomir Visnovsky, a defenseman with the NHL's Edmonton Oilers. Both tested positive for stimulants contained in cold medications.

The IOC conducted more than 2,000 tests in Vancouver, a record for a Winter Games.

The IOC also stores doping samples for eight years so they can be analyzed retroactively once new testing methods become available. If future testing shows an athlete cheated, the IOC can impose sanctions and strip any medals.

The IOC attributed the low number of positive cases in Vancouver to increased pre-games testing by international sports federations and national anti-doping bodies.

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AP Sports Writer Stephen Wilson in London contributed to this report.