The NTSB said Denver's plane - an experimental Long E-Z which he piloted himself - crashed because he took off with too little fuel in one tank, had trouble switching to his backup tank and inadvertently put the plane into a roll.
In a 5-0 vote, the board also blamed the plane's builder for relocating the fuel-tank-selector handle and not placing markings on the handle and nearby fuel gauges.
In addition, the board cited Denver's lack of training in his new plane, which crashed on Oct. 12, 1997, off Pacific Grove, Calif. Denver bought the aircraft just two weeks before the crash and had only limited flying time as its pilot.
The board urged the Federal Aviation Administration and the Experimental Aircraft Association to develop a program under which new pilots of experimental planes would receive formal training before their first flights.
The 53-year-old singer-songwriter was famous for such hits as Rocky Mountain High and Sunshine On My Shoulders.
The plane he piloted plunged 150 feet into the waters off California's Monterey Peninsula.
Witnesses said they heard a sputter, and investigators theorize that Denver ran out of fuel in the left tank and had trouble switching to his right tank.
After hearing from investigators, the board concluded that Denver's plane had only about three gallons of fuel in its left tank when he took off from the Monterey Peninsula Airport in the late afternoon to practice takeoffs and landings. The right tank had only about six gallons, but Denver declined an offer to refuel.