Nick Delpopolo, 23, is the first of the 10,500 games Olympic athletes to fail an in-competition doping test.
His case is the fifth positive test for a banned substance reported by the IOC (International Olympic Committee) since its official London Games testing period began in mid-July.
The other four athletes were caught before competing.
Special coverage: 2012 London Olympic Games
Source: SNTV
Editor: Yao Yuzhi(Intern)
Voiceover: Chris Clark
Producer: Flora Yue