At 2.54pm on Friday, the Clewiston Police Department responded to a call about the incident. A four-year-old boy was found dead at the scene and his five-year-old sibling is in critical condition.
The older sibling is receiving treatment at Hendry Regional Hospital after being airlifted to a trauma centre for treatment.
Clewiston police confirmed a handgun was used in the shooting.
The children were found inside a car.
The children’s parents are “torn up”, according to Hendry County Sheriff, Steve Whidden, but helping investigators.
He said: “We are offering our sincerest condolences to the family of these young children.
“We are determined to provide every resource available to us to conduct this investigation to determine how this tragedy could have occurred.”