Runner Shelby Houlihan, who holds the U.S. record in the 1,500 and 5,000 meters, revealed Monday she was banned for four years after testing positive for a banned substance she says was tainted pork in a burrito.

The 28-year-old made the revelation just days before the start of U.S. Olympic track and field trials. She represented the U.S. in the 2016 Rio Games and finished in 11th place in the 5,000 meters.

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"On January 14th, 2021, I received an email from the Athletics Integrity Unit (AIU), informing me a drug testing sample that I provided on December 15th, 2020 has returned as an Adverse Analytical Finding for an anabolic steroid called Nandrolone and that I am therefore subject to an immediate Provisional Suspension. When I got that email, I had to read it over about ten times and google what it was that I had just tested positive for. I had never even heard of nandrolone," she wrote on Instagram.

"I have since learned that it has long been understood by WADA (World Anti-Doping Agency) that eating pork can lead to a false positive for nandrolone, since certain types of pigs produce it naturally in high amounts. Pig organ meat (offal) has the highest levels of nandrolone.

"In the following 5 days after being notified, I put together a food log of everything that I consumed the week of that December 15th test. We concluded that the most likely explanation was a burrito purchased and consumed approximately 10 hours before that drug test from an authentic Mexican food truck that serves pig offal near my house in Beaverton, Oregon. I notified the AIU that I believed this was the source."

Houlihan added she passed a polygraph test and had some hair samples analyzed by toxicologists.

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"WADA agreed that test proved that there was no build up of this substance in my body, which there would have been if I were taking it regularly," she wrote. "Nothing moved the lab from their initial snap decision. Instead, they simply concluded that I was a cheater and that a steroid was ingested orally, but not regularly. I believe my explanation fits the facts much better — because it’s true. I also believe it was dismissed without proper due process."

She said she was told Friday that her explanation was not accepted by the Court of Arbitration for Sport and she was subsequently banned for four years.

She denied ever taking performance-enhancing drugs.

"I want to be very clear. I have never taken any performance enhancing substances. And that includes that of which I am being accused," she wrote. "I believe in the sport and pushing your body to the limit just to see where the limit is. I’m not interested in cheating. I don’t do this for the accolades, money, or for people to know my name. I do this because I love it. I have so much fun doing it and it’s always the best part of my day."

Houlihan starred at Arizona State and was a 12-time Division I All-American. She won a gold medal at the 2014 NACAC U23 Championships in Athletics and a silver in the 2018 Continental Cup.

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She set her record in the 1,500-meter at the world championships in Doha, Qatar, in October 2019 with a time of 3 minutes, 54.99 seconds. She broke the U.S. 5,000-meter mark with a time of 14:23.92 in Portland, Ore., in July 2020.

The Associated Press contributed to this report.