On July 16, 2000, about 1047 eastern daylight time, a Piper PA-24-250, N980AC, registered to and operated by Naples Air Center, Inc., experienced a loss of engine power and was substantially damaged during a forced landing approximately 5 miles southwest of Immokalee Airport, Immokalee, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 instructional flight. The certified flight instructor (CFI) sustained serious injuries and the private-rated student sustained minor injuries. The flight originated about 1011 from the Naples Municipal Airport, Naples, Florida. The CFI stated that the flight departed with the fuel selector positioned to the right main fuel tank. Airwork was performed, then the student requested to practice patterns. The Immokalee Airport was chosen due to the close proximity. While flying at 1,200 feet, about 5 miles southwest of Immokalee airport, "there was a sudden sharp drop in rpm, checks were carried out, [she] decided to switch fuel tanks from right main to left main but the problem did not clear and [she] prepared for a forced landing." She made a call to indicate their position to a nearby flying airplane and reported her location and intentions. She reported that the propeller stopped "windmilling" and she did not attempt to start the engine using the ignition switch. The landing gear was extended to slow the airplane down, "but the impact speed was around 100 knots." The pilot-rated student stated that the purpose of the flight was for him to get training in complex airplanes. He also stated that he had not been given any formal ground school on the systems of the airplane by the CFI. He performed the preflight to the airplane and noted there was fuel in the right main fuel tank but did not know the quantity. He intended on asking the CFI about the quantity but did not. The CFI arrived at the airplane at they were unable to start the engine; another instructor assisted. Either before or after the engine was started, the CFI placed the fuel selector in the "right main" position. The fuel selector remained on that position for the duration of the flight, "he did not touch the fuel selector at all." When the engine problem was evident, the CFI took the controls; he couldn't recall if she changed the position of the fuel selector valve. He recalled coming in to land in a field that he thought they would make and the last thing he remembers was, "remember specifically, the plane came straight down for last few feet." He could not recall activation of the stall warning system but remembers, "we went straight down." The CFI was on the controls at the time of impact. He recalls that his yoke broke on impact and his seat came off the rails on impact. Examination of the airplane by a mechanic working for the owner and a Collier County Sheriff Department deputy revealed no visible fuel in the right main fuel tank; the fuel selector was found positioned to the right main fuel tank. The remaining fuel tanks were nearly full. Additionally, without NTSB or FAA authorization or supervision, the company mechanic disconnected the fuel line at the inlet of the engine driven fuel pump; no fuel was noted. Following recovery of the airplane, operational check of the fuel selector and auxiliary fuel pump was performed; no discrepancies were noted. A copy of a statement from the mechanic and a copy of the sheriff's department report are attachments to this report. The airplane was equipped with four fuel tanks, two in each wing. The two fuel tanks in each wing are not interconnected. By design to supply fuel to the engine, the fuel selector is required to be positioned to one of the four tank positions. The airplane was fueled last 2 days earlier; it had been operated for 2.4 hours since then at the time of the accident. The accident flight duration was .6 hour.