Very sad. It appears the only witness was a child, so knowing exactly how they all came to be in the water may be tough.

Here's the operating theory from the investigators, which makes sense:

http://www.idahostatejournal.com/news/local/article_666db522-a456-11df-8ac7-001cc4c002e0.html

"Jeffries said the investigation is still ongoing but at this point officials believe one of the men jokingly pushed another into the water. When that person began struggling, one of the other men jumped into the water to try to save him but he also started struggling. The scenario repeated itself as the third and fourth men also tried to save the others."