Interesting article there, and from your link:
"s ISIS or ISIL, began using social media to urge its sympathizers in the United States to stay put and plot violence here.
“They’re targeting the school-shooter types, the mentally ill, people with dysfunctional families and those struggling to cope with different issues,” said one senior law enforcement official ...“We have been pretty successful in disrupting these cases because they are not very sophisticated or smart.”
"......Buying guns is also legal, and investigators have watched nervously as terrorism suspects passed background checks and purchased guns more than 2,000 times in the past decade, according to government data." That is really mind-boggling - is that due to gun laws over there?