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Sam Rivers -- the bassist for the nu metal group Limp Bizkit -- died Saturday ... and now we have more of a clue as to what happened.

A spokesperson for the St. Johns County, Florida Fire Rescue tells TMZ ... they responded to a call Saturday for a "nonresponsive person in cardiac arrest."

The final cause of death is still unclear, but a rep for the St. Johns County Sheriff tells TMZ ... "Deputies responded to an attended death at that address." An "attended death" is one in which the deceased has been a patient under medical care for a serious or life-threatening illness that will likely lead to death in a relatively short period of time -- days or months, but not years. Getty

Rivers temporarily left Limp Bizkit in 2015 ... later revealing it was because he developed liver

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