Kevin of The Smallest Minority has a post up entitled RACIST!™ about U.S. vs. European crime rates & possible causes. It's worth a read.
The only thing I would add is that it is my understanding that culture is the thing we should look at. Culture does tend to follow racial lines, but it is the culture, not any sort of genetic or hereditary qualities that could explain the stats.
But go read Kevin's thoughts on the matter.