What threat does ISIS pose to the homefront?

(CNN) – Yellow, orange, red – the United States has long retired the "rainbow of fear" terror threat alert system that was born out of 9/11.

But the United Kingdom Friday bumped up its threat alert level from "substantial" to "severe," a rating that means that a domestic terror attack is, quote "highly likely."

What has British Prime Minister David Cameron so concerned?

CNN terror analyst Paul Cruickshank and former Department of Homeland Security and CIA official Chad Sweet join CNN to discuss.