Here's a link to a book by Anthony Walsh & Lee Ellis, "Criminology: an Interdisciplinary Approach" and it includes a discussion on IQ. It mentions a 9-14 point gap in IQ between offenders and non-offenders. However, only a 1 point difference between less serious offenders and non-offenders compared to 17 points between serious persistent offenders and non-offenders.

http://books.google.com/books?id=u07ci3cidVsC&pg=PA173&lpg=PA173&dq=prisons+high+IQ&source=bl&ots=uBZpbQjIPX&sig=I_-hiJF0Mxn1cSTFt-4OnRcWgI0#PPA171,M1

The part I found most interesting was in the section of "Intellectual Imbalance". The imbalance is when there is a 12 point difference between VIQ and PIQ. Criminals are significantly more likely to have a VIQ<PIQ imbalance.