The threat of the terror group ISIS blowing up a dirty bomb in the U.S. is not seen as a major issue, yet. 

Terrorism expert Jim Conway says ISIS is trying to build its reputation and recruit more members, competing with al Qaeda.  He doubts whether they could pull it off.

“There's been many attempts, going back to Jose Padilla and other terrorist groups, in an attempt to detonate a dirty bomb,” Conway says.  “They've never successfully detonated one, anywhere in the world.”

Could a dirty bomb be carried into the U.S. over the border with Mexico?  Conway says that southern border is our "Achilles heel" right now, when it comes to potential security breaches.

“They're posturing,” he says.  “They have declared a caliphate in Iraq and in Syria, and they are trying to establish themselves.”