While ice (IMHO) is a prime suspect given the conditions -- like TheHornet says, it could be a chainsaw loose in the prop or a number of other things. Plenty of possibilities for the moment...
I don't normally enter into the condolence thing - but something about this one just pulls at my spirit. Sorry, sorry sorry...for the crew, the pax, the person on the ground, their families, the Colgan extended family, the controller that worked right through it....