It was a US feral government false-flag.
Given the extent to which our liberties have been infringed upon since then, that is not an unreasonable conclusion. The bigger powers play on fear, and fear is easy to stimulate in masses who have no anchor and do not fear God.
I should add at the same time, I am inclined to treat it as an open question. My take is that radical enemies of Western culture planned and carried out the attacks, and the powers that be did not “let a crisis go to waste,” as it were. They seized upon and opportunity to gain a stronger foothold into the deliberate erosion of individual liberties.