During arrival for an ILS-2 approach to runway 11, the pilot was advised to hold at the Kona NDB while snowplows finished clearing the runway. At that time, the Missoula weather was 1200 ft obscured, 2 mi visibility with light snow and fog, temperature and dew point were 32 and 30 deg respectively. About 1 min after reaching the Kona NDB, the pilot declared an emergency and reported that both engines were beginning to lose power. At that time, he was cleared for an immediate approach. Subsequently, the plane collided with trees and crashed between the Kona NDB and the ILS outer marker. During an investigation, all fuel tanks were found ruptured, but fuel stains were found by both tip tanks. No preimpact, mechanical failures were found. Both alternate air handles were found closed. Just prior to crashing, witnesses heard the plane at low altitude. They reported that 1 or more engines were running and heavy snow was falling and blowing.