One athlete who returned to the United States Olympic and Paralympic training centre in Chula Vista as part of a phased reopening of the facilities earlier this month has tested positive for COVID-19.
The Partnership for Clean Competition (PCC) group, which is helping the United States Olympic and Paralympic Committee (USOPC) test athletes for COVID-19, also said one other had already had the virus after returning a positive test for coronavirus antibodies.
The PCC, a non-profit group that funds anti-doping research, said the unnamed infected athlete had been quarantined, alongside those who had "sustained contact" with them, following contact tracing carried out by USOPC officials.
"These tests will give us the first epidemiological profile of COVID-19 in the elite athlete population," said USOPC chief medical officer Jonathan Finnoff.
"We want to know how prevalent the virus is, but we also want to know how an athlete’s body will respond."
Around 25 American athletes returned to the facilities at the beginning of July.
The PCC is aiding the USOPC with its COVID-19 testing programme, which it claims will help guide recommendations for sports organisations as they plan returns to training and competition.
The group has provided the USOPC with $70,000 (£55,000/€60,000) to cover testing for 250 athletes.
"At the end of the day, anti-doping is about athlete health and safety, so while this isn’t our typical area of focus, we felt called to work with the USOPC on this project," said PCC executive director Michael Pearlmutter.
The US has reported the highest number of COVID-19 cases and deaths in the world.
According to latest figures, more than 3.9 million people have been infected with the virus in the country, resulting in at least 144,000 deaths.