PILOT WAS PRACTICING MANEUVERS FOR CERTIFICATION TEST. HE SAID HE MADE A PRECAUTIONARY LANDING DUE TO SUSPECTED ENGINE TROUBLE AND A FIRE STARTED UNDER THE AIRCRAFT IN THE GRASS. HE EXITED THE AIRCRAFT BUT LEFT BATTERY AND RADIO ON. THIS INVESTIGATION INCLUDED REFERENCES TO SABOTAGE FROM A FORMER EMPLOYER, TO ARSON BY THE PILOT. PILOT WAS GIVEN A LIE DETECTION TEST WHICH DID NOT CLEAR HIM. PILOT HAD ALSO LEFT EMERGENCY FUEL SHUTOFF VALVE OPEN. PILOT WAS TESTED IN THE ARSON INVESTIGATION BECAUSE HE HAD BEEN OVERHEARD IN REMARKS "ITS OKAY IF IT BURNS. WE HAVE PLENTY OF INSURANCE" AND "THE BEST THING THAT COULD HAPPEN TO A HELICOPTER IS THAT IT BURN ITSELF UP". NO EVIDENCE WAS RECEIVED FOR IGNITION SOURCE OR FUEL SOURCE BECAUSE OF CONDITION OF WRECKAGE. SOME OF THE PERTINENT VALVES AND LINES WERE DESTROYED. BOTH FUELTANKS WERE INTACT BUT EMPTY.