(Click picture to giant-rolling-snowball-size)
"Wait a minute," (you're saying), "The Riddler?" Well, yes! He got it from Mr. Freeze (one way or another). Man, that's a cold thing to do.
i don't like to contradict the Dark Knight Detective, but his line of detection and supposition all seems a little...frosty. Distracting the police and Batman from his crime scene is not only unusual for the Riddler, it downright breaks his modus operandi. Riddler doesn't distract, he confounds...by leaving a riddle that, if solved properly, will point Batman to the scene of the crime. I'd like to imagine that Batman found this question written in the snow no not that way: "What do you get when you cross a snowball and a wolf?"*
And, I'm assuming that when Batman says "the design had Edward Nigma's thumbprints all over it," I hope he was speaking figuratively and not literally. Because if there were thumbprints on that lovely winter-wielding widget, batman better suspect he's cold on the trail. Why? Because The Riddler always wears gloves.