Ahhhh yes. I'd forgotten the OZ scenario (like the HBO series). The recruits to the cause from the population of Americas toughest prisons. You're correct. Especially those few who might have former military experience or more probably those who have gang affiliations might well pose a problem. Especially if the AQ's promise them right to any "spoils of war" they may come across during the Jihad.
Some of those guys just burn for the chance to get revenge on the "system" and the government. It wouldnt be hard at all for Al Qaeda to recruit them.
A lot would voluntarily join because it would legitimize their lives and they wouldnt have to live with the stigma of being lifelong criminals.
The prison converts could be a huge threat.