You don't know how independent fighting cells operate, either. People are living in a state of denial -- These terrorists can operate within the United States with little or no support from the outside. Everything they need in terms of weapons, cash, intellilgence, etc. can be obtained right here in the U.S. without any assistance from the outside.
This, by the way, is exactly why groups that operate in Israel (Hamas, Hezbollah, etc.) have been very reluctant to do anything here in the U.S. They see this place as the Great Satan, but they also raise a sh!tload of money here. The Irish Republican Army is no different.