Delpopolo said he accidentally ate the drug before arriving in London. He is the first athlete at the London Olympics to fail an in-competition doping test, the AP wrote. Four other athletes were caught before competition began.
He insisted he had not known the food contained the banned substance, the BBC reported.
The International Olympic Committee said Delpopolo "indicated that he was embarrassed by this mistake" and apologized to the United States Olympic Committee and his team-mates.
The BBC wrote the 23-year-old had finished seventh in the men's 73kg competition and was tested after he lost to Mongolian Nyam-Ochir Sainjargal in the repechage stage.