If I recall correctly, he ran out of gas over Monterrey Bay.
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It was theorized that he lost control while switching fuel tanks in one of Burt Rutan’s death traps.
The aircraft John bought second-hand, originating with builder Adrian Davis, was the only amateur constructed Long-EZ with a non-standard placement of the fuel tank select-handle, behind the left shoulder. The aircraft design plans called for placement on the console between the pilots knees. John also failed to conduct a proper pre-flight inspect of an unfamiliar aircraft, and departed the airfield drawing upon a nearly empty fuel tank.
https://www.ntsb.gov/news/press-releases/Pages/NTSB_Determines_John_Denvers_Crash_Caused_by_Poor_Placement_of_Fuel_Selector_Handle_Diverting_His_Attention_During_Flight.aspx
I do not know if it is true, but someone told me that the handle for the fuel valve for the second tank had fallen off, and that he had been using a pair of vise grips to turn the valve. Apparently when he reached back to turn the valve, the vise grips popped off and fell, and after that it was impossible to turn the valve. He may have been trying to get at them or something.