Vigo, Jul 6 (EFE) sources from the Armed Institute have indicated.
The alarm went off when the father went to pick him up at the infant school, which the minor never arrived at.
The day was hot, so the hypothesis is that in the parking lot where he stayed inside the vehicle he could not stand the high temperatures.
The Civil Guard is awaiting the result of the autopsy and an investigation has been opened.
The O Porriño City Council, where the family is from, has decreed three days of mourning, reports a statement.
The flags are already flying at half mast and the Porrigalia festivities scheduled for these days have been suspended.