Police in Delray Beach identified the boy as Khayden Saint Saveur. First responders took the boy to Bethesda Hospital.
"Right now Delray Beach police detectives are investigating to try to determine exactly what happened that led to this child being left in this vehicle", Officer Dani Moschella said.
The child's uncle, Carly Saint Saveur, claimed the auto was parked in the driveway with a dead battery.
A police spokeswoman said it's still not clear how the child got into the auto.
Police found Khayden unconscious when they arrived at the scene and administered CPR.More news: Jamie Murray and Martina Hingis win mixed doubles crown
"It's tragic", said Carly Saint Saveur, the child's uncle. 'I'm scared for him'.
Temperature inside the auto reportedly soared to 65 degrees Celsius in the hot summer day. "If they're sitting in the driveway, hide the keys", Moschella said.
Police said that the inside of a vehicle parked outside in those conditions could climb to 150 degrees within a few minutes. There was one adult at the home at the time, the Sun Sentinel reported.
The toddler, who would have turned 2 on August 24., had been playing with other young family members and may have been in the vehicle for two hours before his family realized he was missing, Moschella said.
At the little boy's house Saturday evening, the cars of family members and friends lined the street and several groups of people stood outside speaking in hushed tones.
'He had lots of energy.