Edited.
I have no doubt that there are any number of terrorist groups that have sponsored "sleeper cells" in the US. We would be incredibly stupid and naive to think otherwise, considering we have been doing the same thing for a century. Why would anyone not think that these groups are well financed, considering much financing comes from America, from the government and from individuals sending money "back home". (I know for a fact that the local Yemeni gas station "owner" sends money home under his American employee's name)
I also would not doubt that there is some rhyme or reason to Yemeni nationals owning gas stations across the US. (probably a number of different countries of origin, but all in the local area are from Yemen)
Someone is sponsoring them, whether it is terrorist-related or not. What kind of problems do you think they might cause if 25% of the gas stations across the nation suddenly stopped selling gas? They don't even have to be "terrorists", have terrorist training, or even have guns. They get the word, they lock the doors, and America is crippled. (the 25% number is a WAG....it could be as much as 40-50%, I have no idea. It is more than 50% in my area, and I'm a little worried)