I was able to do a quick search on my medical journal database. I found an article from UK that states average IQ of inmate there is 87 and many suffer from learning disabilities.

Another study from Chicago prisons compared IQ as a predictor of violent behavior states average IQ for Caucasians as 93.4 and African Americans as 85.4. They considered an IQ of 101 as high at it was a incredibly small group (less than 1%, if I remember correctly) that were qualified as high.

Study from the 80s used BETA I and BETA II IQ test and found a mean IQ of 95.4 among about 6,000 inmates. 2.6% scored 120 and above.

I also found two references that IQ test when repeated after long continuous imprisonment usually yield a lower score due to lack of mental stimulation. The longer you are in prison, the lower the score.

Sorry I can't give you references since these are password protected journals, but I can get pdf's copied if needed.

I didn't see any references to the same information as in the OP. I find that contradicting to many studies in fact.