Can't add much, I'm afraid. A witness told me about it shortly after the accident: apparently the Schweizer had just gotten airborne when it caught fire.
I left Botswana two and a half years ago so I've heard nothing about the outcome of the investigation (the DCA accident reports seem to take a considerable time to surface, even within the local 'trade'). Regardless of who was responsible for the filler cap, you wouldn't really expect a catastrophic result from such a simple (and easily done) error...
Perhaps a websearch for Schweizer-related Airworthiness Directives or Bulletins might throw a bit more light on things. Sorry I can't be of more help.