The six-seater Piper 32, on route from Venice to Leipzig, disappeared from radar at about 11 AM after the pilot reported problems with icing, followed by a Mayday call, Slovenia Traffic Control has told the STA.
The plane crashed in the area of Predmeja above Ajdovščina. On arriving at the scene, the rescue services found the aircraft completely destroyed with the charred body remains in the rubble.
The accident claimed four lives with none of the persons on board surviving, the Nova Gorica police administration said.
The victims, all four men, have been identified as German nationals, police said.
Leipziger Volkszeitung, a German newspaper, reported that one of the victims was 38-year-old Leipzig businessman Thomas Wagner, the founder and owner of Unister, one of the biggest online travel sites in Germany. The paper says the reclusive millionaire was in Venice on business and was returning home.
The investigation was concluded for today and will resume on Friday.
Icing occurs when cold air conditions cause the formation of ice on the surfaces of the plane, which leads to loss of visibility and its aerodynamics.
A single-engine propeller sports plane, Piper 320 is not suitable for flying at low temperatures like those above Slovenia today, Miha Repovž from the Slovenian Aeronautical Association has commented for the STA.
He said the type of plane could fly at altitudes of up to 5,000 metres, but lacked a proper system to prevent the formation of ice on its surface at temperatures around freezing.