On March 15, 2010, about 1120 mountain daylight time, a Migas RV9-A, N951DM, owned and operated by the pilot, nosed over during landing rollout at the Joslin Field-Magic Valley Regional Airport (TWF), Twin Falls, Idaho. Visual meteorological conditions prevailed at the time of the personal flight, and no flight plan had been filed. The amateur built experimental airplane was substantially damaged, and the airline transport pilot was seriously injured. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91. The flight originated from Kennewick, Washington, about an hour earlier. The pilot reported that his traffic pattern approach and touchdown on runway 25 at TWF were normal. Runway 25 is 8,703 feet long and 150 feet wide. The wind was from about 250 degrees at 3 knots, and the runway was dry. According to the pilot, after touchdown the airplane veered "sharply left," and he was unable to redirect the airplane's course back toward the runway using right rudder and right brake. The airplane departed off the left side of the runway while rolling between 25 and 30 miles per hour. The airplane rolled onto an open soft dirt field adjacent to the runway's edge, the nose gear dug into the dirt, and the airplane came to rest upside down. The pilot subsequently examined the airplane. He noted that none of its tires were flat. The nose gear assembly was broken and collapsed. The main wheel brakes were not found locked up. The pilot reported that he had experience landing on grass airstrips as well as concrete runways, and he has no idea why the airplane uncontrollably veered left during rollout. The former airline pilot holds a Federal Aviation Administration (FAA) repairman experimental aircraft builder certificate for his airplane. The pilot also holds an FAA flight engineer certificate and type ratings in four models of Boeing turbojet airplanes. He has over 16,400 flight hours, including 143 hours in the accident airplane of which 32 hours were acquired during the preceding 90 days. The FAA coordinator responded to the accident scene. He reported observing evidence on the runway of nose gear shimmy. The nose gear created a furrow in the soft dirt before breaking in an aft direction. During the FAA coordinator's subsequent interview with the pilot, he reported that no evidence of any control malfunction was detected by the pilot during his flight. The pilot indicated that he normally touches down about 65 knots, and he normally keeps the nose wheel off the ground until slowing to about 40 knots. The pilot reported that the airplane has excellent rudder control at speeds as low as 35 knots. Regarding the accident, the pilot indicated that he does not understand why his airplane veered left or why its nose gear twisted sideways and precluded him from maintaining control during rollout. An FAA certificated aircraft mechanic also examined the airplane. He reported to the FAA coordinator that no mechanical malfunction or anomalies were found with the airplane's flight controls, landing gear, or brakes. No physical evidence related to the pilot’s loss of directional or steering control was found.