 Why the plane veered off its intended route was never established, with highly conflicting opinions from Israeli and Bulgarian investigators.One possibility is that, using NDB navigation, thunderstorm activity in the area might have upset the navigational equipment so that the crew believed they were over the Skopje radio beacon, and turned to an outbound course of 142 degrees, but this version is not supported by any factual evidence of thunderstorms in the area.As a result, this version of events is disputed by both the Bulgarian military and current historiographers of Bulgarian aviation.