Three National Park Service employees -- Mason McLeod, 26, Neal Spradlin, 28, and Seth Spradlin, 20 -- were on the de Havilland Beaver when it vanished. The plane was piloted by 47-year-old Marco Alleto.
The plane was on a short flight over rugged terrain between Swikshak Lagoon and King Salmon.
Searchers found no sign of it after planes repeatedly criss-crossed the region looking for it, and last week the official search was called off.
The father of McLeod, Steve McLeod, a retired police officer who traveled to Alaska from Florida, said Monday that the reward has been increased from $15,000 to $65,000.
Anyone with information about the plane is asked to call the National Park Service at 907-246-3305, or the Alaska State Troopers at 907-246-3346.
"The families ask that you do not endanger yourselves or anyone else in your efforts to find our children," McLeod said in an e-mail Monday.
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