At 1520 CDT, a Piper PA-31 Chieftain lost power and made an off-airport landing. None of the seven aboard were injured. The aircraft had just taken off from Mt. Comfort Airport with a destination of Waterloo, Ontario, Canada. The flight crew said the engine start and run-up were normal but that they experienced a gradual loss of engine power on both engines shortly after takeoff. Investigators determined that the aircraft was refueled with Jet-A instead of avgas at the Mt. Comfort Airport.