Police Capt. Jeff Baker tells the Times Herald of Port Huron that investigators believe the boy climbed into the car and succumbed to the heat.
Temperatures were in the 70s Friday in Port Huron, about 55 miles northeast of Detroit.
Derrick Holman had Down Syndrome. Port Huron Police Sgt. Dave Seghi said the boy's parents last saw him playing about 8:30 a.m. Friday. He was reported missing about 11 a.m. His body was found about seven hours later.
Officers from a number of area departments were joined by a state police helicopter in the search.
Baker said "it's heartbreaking for it to end this way."