Mystery Bird Imprint in the Snow (ID challenge)

I was walking along Milwaukee's lakefront yesterday when I came across the imprint of a bird in the snow. It looked like the bird had hit the snow directly from above, leaving impressions of its body, wings, tail and part of the head. This bird then apparently took off vertically. There were no signs of the bird walking or struggling away, and no signs of another animal dragging it away. The snow all around the imprint was undisturbed. That made me think it was the work of some bird of prey.

Here's my original photo of the imprint (there had been some drifting of the snow since the imprint was originally made):

http://bit.ly/hBRNLw

I measured the imprint by taking photos of my shoeprints alongside the imprint. It looks like the wingspan was maybe 29 inches. The length of the body from tip of tail to approximate top of head was 17-18 inches. The width of the tail (spread) was about 6 inches at the tip. The length of the tail was maybe 4 inches.

Here's a photo of the imprint with my shoeprints alongside. My shoes are just shy of 12 inches long:

http://bit.ly/eErenk

Any ideas about what bird made this imprint?