About 20 athletes have become sick after participating in a triathlon earlier this month that included a swim in the Oklahoma River, which has had problems with fecal coliform pollution, state health officials said Tuesday.

It is too early to determine the cause of the illness, symptoms of which included diarrhea, vomiting, fever and abdominal pain, Oklahoma State Department of Health spokeswoman Leslea Bennett-Webb said. She said she didn't know if anybody had been hospitalized.

"It certainly could have been the river," she said, "but we don't know that yet at all."

She said participants in the Boathouse International Triathlon, held May 16-17, should contact their personal physician if they have any symptoms and the doctor should call the Health Department. Bennett-Webb said 367 athletes participated in the event, the first of its kind along the river.

She said health officials are also trying to contact athletes to see if they are having any symptoms.

Jenna Shoemaker, an elite triathlete from Boston who finished second in the event, told The Associated Press on Tuesday that "today is the first day my stomach feels normal again. It was a good week of being sick. ... My body was very achy, as if I'd been run over by a couple of trucks."

The event, which includes swimming, cycling and running, began with a 1.5-kilometer swim in the Oklahoma River, starting at the docks by the Chesapeake Boathouse near downtown. The river, formerly a ditch that handled runoff, has been transformed in recent years into prime venue for rowing events, but this was the first time a triathlon was held there.

Mike Knopp, the Chesapeake Boathouse's executive director, said he learned of the reports of gastrointestinal illness over the weekend and is waiting to learn more about what caused the problem.

"I think that it's one of those things that could be a lot of things," he said. "We're just waiting to hear."

Shoemaker said she's living proof that it was the river water that caused her and other competitors to become sick, because she didn't stay at the hotel with other competitors and didn't consume the same food and beverages they did, yet still became ill.

"It definitely came from the water," she said.

Scott Schnitzspahn, the sport performance director for Colorado Springs, Colo.-based USA Triathlon, said about six elite U.S. triathletes became sick, and about 10 elite and junior competitors from Canada also fell ill. He said the illnesses lasted from the Sunday after the race to "about mid-week for most, some into this past weekend."

Schnitzspahn said that "while this sounds significant, the rest of the athletes did not get sick at all and had a great experience in Oklahoma City."

Although tests on the river water were done for about six months before the event, Knopp said no testing was done immediately before the event. The processing time for such tests make it impractical to conduct them right before an event, he said.

"You can't turn a test that quickly," Knopp said. "That's the problem."

He said previous testing indicated the river's water quality was generally good for swimming but could vary, depending on weather events.

Todd Brewer, the city of Oklahoma City's water quality lab manager, said the bacteria sample seemed elevated in water samples taken from the river after big rain storms. Parts of Oklahoma City received up to two inches of rain the day before the event.

"We're learning as we go," Knopp said. "We certainly don't want to get anybody sick."

In an AP story before the triathlon, Derek Smithee, the water quality division chief of the Oklahoma Water Resources Board, said a six-mile section of the river near the boathouse is listed by the state as "impaired" because of fecal coliform, sulfates and turbidity.

Smithee said the ranking didn't mean the water wasn't safe for swimming.

"It's probably more dangerous to drive to the Oklahoma River than to swim in the Oklahoma River," Smithee said at the time. He did not respond to calls for comment Tuesday.

The river opened to the public in December 2004. According to the city's Web site, swimming is not allowed in the river.

Shoemaker said "it's terrible that this happened" because the competitors enjoyed their time in Oklahoma City. She praised the race organizers, but said that "unfortunately after this, I don't think even if they held an event ... that many people would go back."