Washington’s Blog 
Sept 8, 2012
In December, an American court entered a default judgment against Iran  for allegedly having a hand in the September 11th attacks. 1
On the other hand, a 9/11 Commissioner and the Co-Chair of Congressional Inquiry into 9/11 both say in sworn declarations that the Saudi government is linked to the 9/11 attacks . 2
Then again, right after 9/11, Bush, Cheney and Rumsfeld accused Iraq of having a hand in the attacks .
People may not remember now, but – at the time – the supposed Saddam-9/11 link was at least as important a justification for the Iraq war as the alleged weapons of mass destruction . 3
Of course, the Afghanistan war was largely based upon the allegation that the leaders of that country – the Taliban – had harbored Bin Laden and other Al Qaeda terrorists. In other words, the U.S. government treated Afghanistan as being complicit in 9/11 by giving refuge to the 9/11 attackers. 4
At a minimum, America unintentionally created an atmosphere in which Al Qaeda terrorists could thrive . But numerous former U.S. military and intelligence officers think  that rogue elements within the U.S. government intentionally allowed or facilitated  the attacks.
A government official told  Fox News:
Evidence linking these Israelis to 9/11 is classified. I cannot tell you about evidence that has been gathered. It’s classified information.
While many people say that Israel only spied on the hijackers – and didn’t withhold actionable intelligence from the U.S. government – others, including some former military officials (like this guy ), allege that Israel withheld information from the U.S. or even had a hand in the attacks , pointing to suspicious activities regarding moving vans and explosives on 9/11. See this , this , this , this  and this .
What does this mean? We don’t know for sure which nation or nations backed the 9/11 terror attacks.
But as the Co-Chair of the Congressional Inquiry into 9/11 and former Head of the Senate Intelligence Committee Bob Graham told PBS Newshour  in 2001, 9/11 was state-sponsored terrorism, and it is vital for America’s national security that we get to the bottom of who the backing country was (and whether more than one country was involved):
[Question]: Senator Graham, are there elements in this report, which are classified that Americans should know about but can’t?
SEN. BOB GRAHAM: Yes … I was surprised at the evidence that there were foreign governments involved in facilitating the activities of at least some of the terrorists in the United States.
I am stunned that we have not done a better job of pursuing that to determine if other terrorists received similar support and, even more important, if the infrastructure of a foreign government assisting terrorists still exists for the current generation of terrorists who are here planning the next plots.
To me that is an extremely significant issue and most of that information is classified, I think overly-classified. I believe the American people should know the extent of the challenge that we face in terms of foreign government involvement. That would motivate the government to take action.
[Question]: Are you suggesting that you are convinced that there was a state sponsor behind 9/11?
SEN. BOB GRAHAM: I think there is very compelling evidence that at least some of the terrorists were assisted not just in financing — although that was part of it — by a sovereign foreign government and that we have been derelict in our duty to track that down, make the further case, or find the evidence that would indicate that that is not true and we can look for other reasons why the terrorists were able to function so effectively in the United States.
[Question]: Do you think that will ever become public, which countries you’re talking about?
SEN. BOB GRAHAM: It will become public at some point when it’s turned over to the archives, but that’s 20 or 30 years from now. And, we need to have this information now because it’s relevant to the threat that the people of the United States are facing today.
1 A default judgment – such as the one against Iran – means that the defendant doesn’t appear in court, and so does not present a defense or contrary evidence. The judge instead often accepts virtually whatever the plaintiff claims, without examining or considering any contrary evidence.
2 The 9/11 Commissioner and the Co-Chair of Congressional Inquiry into 9/11 who say that Saudi Arabia is behind the 9/11 attacks are calling for either a “permanent 9/11 commission ” or a new 9/11 investigation  to get to the bottom of it.
3 This claim that Iraq is linked to 9/11 has since been debunked by the 9/11 Commission , top government officials, and even – long after they alleged such a link – Bush  and Cheney  themselves.
4 The Taliban allegedly offered to hand over Bin Laden to the U.S. , but the U.S. refused the offer, and started bombing instead.