The Register-Guard reports the NTSB says witnesses saw the plane descend from the clouds, then heard an increase in engine power. They say the plane then stalled and began to spin before it hit the water northwest of Florence on June 15.
Killed in the crash were 69-year-old Richard Munger of Florence and his grandson, Benjamin Dressler of Burien.
The report says Munger's Grumman American AA-5B crashed about an hour after takeoff from the Florence Municipal Airport.
The NTSB says plane components are being recovered from the ocean as part of the ongoing investigation. The board will release additional details after it determines the probable cause of the crash.