Wow...I'm from the Chicagoland area and I sort of remember hearing a news report a couple of years ago about wild dogs attacking someone (a jogger maybe?) at a Cook County (the county Chicago is in) forest preserve. I think people were just dumping unwanted, probably agressive dogs there.
Okay...I just found a news link. A woman was actually killed by one of the dogs while jogging.
http://abclocal.go.com/wls/story?section=News&id=160453