On March 9, 1996, at 1700 central standard time, a Piper PA-28-140, N8726N, registered to a private owner, operating under Title 14 CFR Part 91, impacted trees and terrain following a total loss of engine power near Gladewater, Texas. Visual meteorological conditions prevailed for the planned local personal flight and a flight plan was not filed. The private pilot and passenger were not injured and the airplane sustained substantial damage. The flight originated from runway 31 at the Gladewater Municipal Airport about 3 minutes before the accident. On the enclosed Pilot/Operator report, the pilot stated the following information. Beyond the departure end of runway 31, during the initial climb, the pilot noticed "cold air" from the left air vent and "adjusted [the] air vent." Approximately 30 to 45 seconds later engine power was lost. During the emergency procedure, the "engine regained RPM;" however, before a 180 degree turn around was completed, the "engine lost power." At this point, clearance above trees was less than 30 feet and the "aircraft stalled at tree top" height. The airplane came to rest nose down. Damage occurred to the wings, nose gear, and firewall. During personal interviews, conducted by the FAA inspector, the owner, and local authorities, the pilot reported that he thought he inadvertently turned the fuel selector to the "OFF" position. The owner reported that he and local authorities examined the airplane and found the fuel selector in the "OFF" position and the left vent open.