Mississippi Valley Airlines Flight 106 departed Minneapolis at 15:25 for a flight to Winona Municipal Airport, MN (ONA) and climbed to its assigned cruising altitude of 5000 feet. Shortly after departure the flight was advised that weather at the destination was below minima. Minneapolis ARTCC then cleared the crew to La Crosse. Weather at La Crosse was reported being indefinite ceiling, 500 feet obscuration, visibility half a mile, light drizzle and fog. Shortly after 4 pm the flight passed the Holman Intersection at 3000 feet msl, descending to the Midway final approach fix which was crossed at 2100 feet. The pilot leveled the plane at 1100 feet. Shortly thereafter, the plane struck trees. Both wings, the engines, the left main landing gear, strut an left horizontal stabilizer separated from the aircraft. PROBABLE CAUSE: “For reasons unknown, the captain failed to maintain altitude at minimum descent altitude and allowed the aircraft to descend below the height of the trees while executing a nonprecision instrument approach in instrument flight conditions.”