Ethnicity and/or race doesn't seem to be the factor. Lindh, Reid, and this guy prove that non-Arabs can be terrorists intent on killing Americans, Islam seems to be the main factor in this.
EXCEEDINGLY difficult to research, but I suspect if you looked at the total Muslim population in the US (all Muslims, not just prison converts, which is an obviously self-selecting population) that the rate of crimes committed by Muslims in the US is lower than that committed by those baptised as Christians. (Of course, people cleverly get around that by instantly proclaiming that anyone baptised as a Christian or who ever attended Christian services who ever does anything bad instantly STOPS being a Christian at that point.)