Assumption Parish Sheriff Mike Waguespack told CNN the alligator hunter appeared to suffer from a seizure and fell backward Monday on the Intercoastal Waterway. Attempts to revive Guist, 47, were unsuccessful and he was pronounced dead Monday at Teche Regional Medical Center in Morgan City.
Waguespack told E! News Wednesday the preliminary report from an autopsy performed on the reality TV star's remains showed Guist died of natural causes.
"More details will be released when the final report is made" in four to five weeks, he said.
E! News said it was unclear whether Guist's body has been released to his family yet.
"Right now, we're thinking about him almost every minute," Brian Catalina, executive producer of The History Channel's "Swamp People," told CNN. "We've lost a really important part of our family and a treasured friend."
Guist starred on the show alongside his brother, Glenn.
"The two were inseparable. These guys were born in the same house that they both still lived in, up until yesterday. They were two peas in a pod for sure. Neither had ever married. They were just as brothers as you could be," Catalina said.