Late Limp Bizkit Bassist Sam Rivers' Emergency Call Came in as Heart Attack

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.

Rivers temporarily left Limp Bizkit in 2015 ... later revealing it was because he developed liver disease due to excessive drinking. He received a liver transplant and rejoined the group in 2018.

The group announced the news of his passing Saturday evening, calling him their "heartbeat."

Sam wasn't just a bass player, the group said -- he "was pure magic. The pulse beneath every song, the calm in the chaos, the soul in the sound."

Rivers was 48.