The man's 16-year-old son has been arrested in the death.
Police say an autopsy done by the Fond du Lac County medical examiner shows the preliminary cause of death as asphyxiation. Toxicology tests are pending.
Officers received a call about 3:30 a.m. Sunday about a vehicle with a body inside at Lakeview School just outside the city.
The teenager was taken into custody about five hours later at a Neenah apartment. A juvenile girl at the apartment also was taken in for questioning.

