I would guess that 9 out of 10 Japanese do not have a hard line on who perpetrated 9/11.
I think if asked who did it they would reply that it was al-Quaida and bin Laden. But if you asked if it could have been factions in the U.S. government I doubt they would dismiss it out of hand.
You see, its a very complicated question for which we may never see any actual proof from the very people who are suspect in the question. The U.S. government is honest with us about precious little. And the past has proven that its pretty much capable of anything.
And what does it mean "to perpetrate" anyway? Does it have to be the guy with his hands on the wheel? Does it include the person who ordered it? The people who instigated it? The people who nudged it along? The people who knew it was coming but did nothing?
My personal feeling (and without proof I could never have, that is all I can get is a feeling) is that certain persons in the Bush admin, CIA, FBI and Israel's Mossad (at least) knew it was coming. And saying that, I am not ruling out possible assistance given to the physical perpetrators at least on the sly, even without direct contact to the physical perpetrators.
Of course that feeling may primarily be founded on the way certain people in the government have so deftly and disgustingly capitalized on the the 9/11 attacks, but still, I cannot shake the feeling that the government knows a lot more than its telling us and its because of government involvement in some capacity.
And before you scream "Truther!" remember that even the good guys of the 9/11 commission accused the CIA, FBI and Bush Admin of obstruction of their investigation. Even the FAA and NORAD handed them a bunch of baloney. How anyone could just write that off has little explanation in my mind beyond blatant stupidity. Perhaps a misguided sense of patriotism and/or a need to for authority to guide their sheepish lives?
Anyway, the 9/11 commission was well underfunded in the beggining and did not get proper financing until well after memories had dried up and evidence disappeared. And when the commissioners come out and tell you that their report was the only report they could make because of evidence they themselves were denied, I think that should tell you that things are not on the up and up.
Criticism of the 9/11 Commission - Wikipedia