Story highlights U.S. Rowing: It would be "easy but irresponsible" to assume the rowing course is source of illness

Concerns have previously been raised over water quality in Rio's Rodrigo de Freitas Lagoon

(CNN) Four coaches and 11 members of the U.S. rowing squad fell ill after taking part in a pre-Olympics test event in Brazil's Rio de Janeiro, amid ongoing concerns over water quality ahead of next year's Games.

The squad was taking part in the 2015 World Junior Rowing Championships, held at the weekend on the Rodrigo de Freitas Lagoon in the heart of Rio.

U.S. Rowing said Tuesday it was looking into the cause of the illness and that it was not possible to say what was to blame.

However, incidents in which dead fish have washed up in the lagoon -- also the venue for Olympic canoeing events -- have previously prompted questions over the safety of the water.

Alarm bells have also sounded over pollution in Guanabara Bay, site of the Olympic sailing events, and Copacabana Beach, where triathletes dived into the surf earlier this month for another pre-Games test event.

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