THE ACFT COLLIDED WITH TREES ON UNLGTD TERRAIN AT NGT AFTER MAKING A LOW PASS AT APRX 60 FT AGL WITH THE GEAR EXTENDED. WITNESSES REPORTED THAT THE ACFT'S CLEARANCE LIGHTS WERE NOT OPERATING UNTIL THE RWY LIGHTS WERE TURNED ON, THEN THEY OBSERVED THE ACFT'S STROBE LIGHT. THE RWY LIGHTS COULD HAVE BEEN TURNED ON BY KEYING THE RADIO MIKE ON 123.45 MHZ. THE WITNESSES HEARD THE SOUND OF THE CRASH WHICH WAS FOLLOWED BY SILENCE. DURING IMPACT IN A WOODED AREA, THE ACFT COLLIDED WITH SEVERAL TREES OVER AN AREA OF ABOUT 175 FT. THE ACFT'S LIGHT & ALTERNATOR SWITCHES WERE FOUND IN THE 'OFF' POSITION; THE BATTERY SWITCH WAS ON. THE ALTERNATOR PULLEY HAD A FRACTURE SEPARATION; HOWEVER, THE MODE OF SEPARATION WAS NOT DETERMINED. THE ACFT HAD DEPARTED MANSFIELD, MASSACHUSETS FOR MANSFIELD, NEW HAMPSHIRE AT 1845 EST. THE PLT HAD PLANNED TO RETURN IN 4 HRS.