IMO, most of the 'terrorist' attacks in the US were the work of an ad hoc team of US government operatives and contractors. This includes the WTC attacks (both 1993 and 2001), Oklahoma City, San Bernardino, Orlando, Sandy Hook, Las Vegas, and Parkland.
People who think their gov't would never mistreat or harm it's own citizens are living in a very dark, deep cave of ignorance, denial, apathy, and/or cowardice. The only way to keep our gov't relatively honest is for a critical mass of the citizens to come out of their caves and confront the problems any way they can. Once the gov't realizes it's losing its illusion of respectability, the tide will turn.