RIO DE JANEIRO (AP) — 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 reported some illnesses, according to World Rowing, the sport’s governing body, but those were about as expected at an event that featured more than 500 young rowers.

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 Lake, 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.

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.”

US Rowing, which oversees the sport in the United States, said it is investigating what sickened the athletes, who range in age from 16-19. None are likely to be Olympians next year.

Rowing officials will debrief the athletes when they return to the U.S., likely through the rest of the week. They will talk to the athletes, review protocols for cleanliness.

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.

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.

The AP analysis of water began in March and was performed by noted Brazilian virologist Fernando Spilki, coordinator of the environmental quality program at Feevale University in southern Brazil. It showed dangerously high levels of viruses from sewage in all Olympic venues. The samples were checked for three types of human adenovirus, as well as rotavirus, enterovirus and fecal coliforms.

These are viruses that are known to cause digestive and respiratory illnesses, including explosive diarrhea and vomiting, but can also in rarer cases lead to more serious heart, brain and other diseases, such as hepatitis A.

The AP testing, which will continue through the Olympics, also checked for bacterial fecal coliforms — which at times during the study peaked at the Olympic lake to 10 times the acceptable limit for secondary contact per Brazilian regulations.

In two separate emailed statements following the AP study, the World Health Organization affirmed it was advising the International Olympic Committee “to widen the scientific base of indicators to include viruses.” The WHO underscored that it’s actually up to the local Olympic organizing committee in Rio to order that viral testing be done.

However, in an emailed statement Monday, the organization reversed course and said that “WHO has not and will not issue an ‘official recommendation’ on viral testing.”

The local committee has not responded to repeated requests for information on whether it will order the viral testing.