NTSB Probable Cause

IMPROPER PLANNING/DECISION BY THE PILOT, THE PILOT'S FAILURE TO MAINTAIN PROPER ALTITUDE AND CLEARANCE OVER MOUNTAINOUS TERRAIN, AND THE COPILOT'S FAILURE TO ADEQUATELY MONITOR THE PROGRESS OF THE FLIGHT. FACTORS RELATED TO THE ACCIDENT WERE: INSUFFICIENT TERRAIN INFORMATION PROVIDED BY THE FLIGHT SERVICE SPECIALIST DURING THE PREFLIGHT BRIEFING AFTER THE PILOT INQUIRED ABOUT A LOW ALTITUDE DEPARTURE, DARKNESS, MOUNTAINOUS TERRAIN, BOTH PILOT'S LACK OF FAMILIARITY WITH THE GEOGRAPHICAL AREA, AND THE COPILOT'S LACK OF FAMILIARITY WITH THE AIRCRAFT.