After departure, the airplane intercepted airway V-2, and appeared to climb normally until reaching 8300 ft (9000 assigned) while tracking approximately 110 deg mag. Radar analysis showed the airplane at 128 KIAS when it abruptly turned left 30 deg and then appeared to recover. Voice communication during this event was normal with no change in the pilot's level of anxiety. Immediately thereafter, the heading changed over 90 deg to the left (northbound), and a maximum 6750 fpm rate of descent developed before the heading stabilized and the descent slowed. The airplane's ground impact site was approximately 1 nm southwest of the last radar target. Wreckage distribution was roughly parallel to V-2. The airplane impacted the mountainside in a steep nose low, left wing down attitude. The engines, propellers/governors were disassembled and inspected with no evidence of mechanical malfunction. Examination of the left propeller indicated low power on impact. There was no evidence of significant icing or turbulence.