1/ 14 years have passed since 9/11. Why has American public opinion never raised serious questions about the main cause of the incident?

Whether or not al-Qaeda and Bin Laden were the real architects of 9/11 remains disputed. The unwllingness to carry out a full investigation of 9/11 is inexcusable, and contributes to the uncertainty about who was behind the attacks. In a 2008 World Public Opinion poll, about half the world believe the official version of al-Qaeda carrying out the 9/11 attacks, a quarter don’t know, and a quarter believe it was a conspiracy by some combination of the US, Israel and other Arabs.
People in the Middle East were especially likely to name a perpetrator other than al-Qaeda. Not surprisingly, Israel is seen as a likely perpetrator among Egyptians (43%), Jordanians (31%), and Palestinians (19%). The US government was named by 36% of Turks and 27% of Palestinians. Among Europeans only Germans (23%) and Italians (15%) accuse the US government, and almost no one accuses Israel.
Among Americans, according to a YouGov poll in 2013, 40% believe the official explanation, 38% “have some doubts”, 12% are unsure, and 10% “do not believe it at all.” In yet another poll, 11% of Americans believe it was actually the US government.
By factoring in the don’t knows, we can say roughly speaking that half the people around the world believe the official story attributing 9/11 solely to al-Qaeda (apart from Egyptians and Jordanians), about a quarter believe it was some complex conspiracy using Arab patsies manipulated by the US and/or Israel, and a quarter don’t know.
If some secret faction within the US and Israel governments was the active instigator, that would indeed account for the stonewalling in the investigation, but it is hard to believe that in the era of wikileaks and brave whistleblowers such as Edward Snowden and Preston Manning, that some concrete evidence has not been made public. Whistleblowers point to ignored FBI and CIA warnings, but this evidence, and memoirs by such as CIA head George Tenet and White House counterterrorism czar Richard Clarke, tend to undermine claims of wilful conspiracy by US government leaders. The only possibility is that a "deep state" faction of the US-Israeli elite carried it out, miraculously avoiding any leaks.

2/ What is the most important  document which points to Israel and the US?

Available evidence, and the many contradictions and inconsistencies surrounding the official 9/11 story, suggest that at least some US, Saudi, Pakistani and Israeli officials were aware that a major attack was in the offing in the US on 9 September 2001 using al-Qaeda operatives.
Officially the 19 Arab youth (15 of them Saudi), were operating under their al-Qaeda mentors (notably, Osama bin Laden, Khalid Sheikh Mohammed and Yusuf bin al-Shibh). While ‘deep state’ conspiracies and false-flag attacks by the CIA and Mossad5 do take place, this event seems just too complicated to conceive of as a ‘deep state’ conspiracy, though that possibility remains until convincing answers to the many discrepancies in the official explanation are given.
Evidence censored from the Joint IntelligenceCommittee Inquiry of 9/11 (2002)—the 28 redacted pages— was leaked to the US press by ex-Senator Bob Graham, pointing to Saudi officials’ involvement in 9/11 and suggesting a scenario involving Pakistan’s ISI and rogue (?) Saudis. Some of the actors revealed in the 28 pages (so states Graham, who is among the significantly few officials who have actually seen the pages) include Pakistani and Saudi officials and nationals, including Pakistanis Lieutenant General Mahmud Ahmed and Saudi officials Omar al- Bayoumi, Osama Basnan, Princess Haifa (wife of Saudi US ambassador Prince Bandar), and Saudi businessman Abdulaziz al-Hijji. They all had repeated, direct contact with those directly involved in the hijackings.
Consider Pakistani Inter-Services Intelligence (ISI) Director General Mahmud Ahmed, who was ‘retired’ on 8 October 2001 by Pakistani dictator Pervez Musharraf. The reason? The next day, The Times of India reported that, “US authorities [FBI] sought his removal after confirming the fact that $100,000 were wired to WTC hijacker Mohamed Atta from Pakistan [in fact, the UAE] by British-Pakistani Ahmed Omar Sheikh at the insistence of General Mahmud”. Mahmud Ahmed is now working for the Tablighi Jamaat dawa society, proselytizing Islam. The 9/11 Commission’s Final Report states that the source of the funds “remains unknown”.
Ahmed Omar Sheikh—who transferred the $100,000 to Atta several weeks before 9/11 and received approximately $8,000 (the unspent funds) from Atta a few days before 9/11—was arrested by Pakistani police in February 2002 for the Daniel Pearl kidnapping, sentenced to death, and has spent the subsequent 11 years incommunicado. It appears now that he was not Pearl’s murderer (Khalid Sheikh Mohammed confessed to this after his arrest), but Pakistan refuses to permit his extradition to the US in connection with Pearl or 9/11, presumably because he could implicate senior Pakistani officials.
Ex-Senator Graham, who chaired the Joint Inquiry in 2002, told IBTimes in 2013: “The Saudi government without question was supporting the hijackers who lived in San Diego. You can’t have 19 people living in the United States for, in some cases, almost two years, taking flight lessons and other preparations, without someone paying for it. But I think it goes much broader than that. The agencies from CIA and FBI have suppressed that information so American people don’t have the facts.”
Whether or not the legendary 28 redacted pages are made public, the pieces of the puzzle are falling into place. Pakistan’s ISI and rogue Saudis were directly involved in planning and financing 9/11. The idea was/is a Saudi-led hegemony in the Muslim world, where Pakistan would preside in Central Asia. The Pentagon, CIA and FBI lied to the 9/11 Commission, not only to protect their links to their own duplicitous agents, but—more importantly—to prevent the whole Saudi/ Pakistani/ US alliance from collapsing in a real Islamic revolution, a la Iran. (This is what was beginning to happen with the rise to power of the Muslim Brotherhood in Egypt, which the Saudis sabotaged with US support.)

3/ Given the current developments in the middle east, can it still be said that the event happened for the benefit of the US?

On the surface, the western project to forcibly reform the Muslim world to meet the needs of imperialism—the ongoing 'Great Game'—received a Godsend in 9/11. It was as if Bin Laden, the supposed perpetrator of the collapse of the World Trade Center, was in cahoots with the US neocon political establishment, and had provided them on demand with the pretext for a war on Islam.
US president Bush quickly moved to take advantage of the situation, invading Afghanstan and Iraq, creating chaos in the middle east, blind to the repercussions of invading countries. The deaths of US soldiers pales in comparison to suffering of those nations, but the huge costs of the invasions financially and the tens of thousands of US military casualties puts a huge burden on the US, leaving its imperialist plans unfulfilled and unacceptable by other nations, even the staunchest allies of the US.

4/ Would you say the rise of extremist, terrorist groups including ISIL in the region is a result of the 9/11 attacks?

Destroying Afghanistan as a base for al-Qaeda and finally killing Bin Laden did not end the threat of terrorist acts, but transformed their context. Disabled as a centralized organization with the ability to function around the world, now the new generation of self-styled jihadists, calling themselves al-Qaeda and now ISIL, carry forward its long term plan for the region, ‘managing savagery’, without any direct relation with the remaining members of Bin Laden’s original organization. All the causes of the rise of terrorism from the 1970s on—extreme poverty and endemic unemployment, in the face of humiliation by US-Israeli hegemony in the region—are still valid, more so since the 2013 coup in Egypt. In light of this, we are caught in the “intervention trap” (invasions and bombings merely give further cause to rebels), and the cycle of violence will continue.

http://kayhan.ir/fa/issue/527/12