Thirteen rowers on the 40-member U.S. team came down with stomach illness at the World Junior Rowing Championships — a trial run for next summer's Olympics — and the team doctor said she suspected it was due to pollution in the lake where the competition took place.

The event took place amid rising concerns about the water quality at venues for the Rio de Janeiro Olympics, now less than a year away.

The Americans were by far the hardest hit at the regatta that concluded over the weekend, with reports of vomiting and diarrhea. Other teams in the competition also reported some illnesses, according to World Rowing, the sport's governing body.

On July 30, The Associated Press published an independent analysis of water quality that showed high levels of viruses and, in some cases, bacteria from human sewage in all of Rio's Olympic and Paralympic water venues, including the Rodrigo de Freitas Lagoon, where the rowing competition took place.

U.S. coach Susan Francia, a two-time Olympic gold-medal rower, said in an interview with the AP that 13 athletes and four staff members — including herself — suffered various gastrointestinal symptoms during the team's two weeks of training in Rio.

However, U.S. and international officials said there was no evidence that abnormal levels of viruses or bacteria in the water caused the rowers to fall ill, and that the health issues they experienced are not unusual in international events.

“It would be easy but irresponsible for us to immediately assume that the rowing course is the main or sole point of exposure that caused the illnesses,” said Glenn Merry, CEO of U.S. Rowing, which oversees the sport in the United States.

Merry added that U.S. athletes often took ill abroad and said the fact that coaches also got sick in Rio was an indication water might not be the problem.

Dr. Kathryn Ackerman, the U.S. team physician, said athletes from several other countries stayed in the same hotel as the Americans, but did not seem to get as sick as her rowers.

“I don't know if it was the water bottles in the boats, or hygiene precautions that some athletes are really good about and others weren't,” she said.

Officials did not rule out that the Americans could have gotten ill from food or drinking water.

“We're not really sure. My personal feeling is, I think it's from the lake,” Ackerman said.

Francia said she lacked the data and information to directly blame the illness on the venue, but added: “It just doesn't seem normal.”

She warned athletes coming for the Olympics that “you should know when you're coming next year that you have to be smart about how you are preparing.”

Francia said the U.S. team had taken precautions about competing in the polluted lake beneath Rio's picturesque Christ the Redeemer statue, “but maybe we were not as strict in enforcing them as we should have been from the beginning.”

“As soon as kids started going down, we were bleaching oar handles, we were immediately washing hands after coming off the water,” she said. “Other countries didn't allow water bottles at all. Other countries had water bottles in zip-locked bags.”

U.S. Rowing said it is investigating what sickened the athletes, who range in age from 16-19. None is likely to be an Olympian next year.

Ackerman said she became worried when one U.S. boat tipped over in the lake, although the athlete who got thrown into the water was not among those who became ill.

“Obviously we were all concerned because we know the water's polluted,” she added.

A spokesman for the Rio organizing committee on Monday attributed American team illnesses to “travel symptoms” and said an event doctor treated eight Americans, three Britons and three Australians for symptoms including diarrhea.

The Americans' experience is almost certain to raise more concerns for the Olympics. About 10,500 athletes will attend the Summer Games, and 1,400 will participate in rowing, sailing, triathlon, canoeing and distance swimming in the waters around Rio.

“You don't want to see athletes in the boat-park vomiting,” Francia said, recounting that the competitor she saw get sick was not an American. One of the U.S. rowers did faint in a dining area, she added.