There are a lot of conspiracy theory regarding the 9/11 attack about involvement of some US agencies or groups.
I don't think those conspiracy theory holds any truth but them seem to be right in another way.
The royal family of Saudi, one of the largest trading partner and strategic and military ally of USA played a major role in funding the 9/11 attack which again is the money of USA traded for Saudi oil.
The so-called “28 pages” suggest a much larger web of connections between al-Qaida and the Saudi royal family than had previously been known.
Even as the White House – and the Saudi ambassador to the US – insisted that the 13-year-old report did not implicate senior Saudi officials in supporting al-Qaida, family members of the 9/11 victims who have long demanded the report’s release, as well as some of their congressional allies, said they believed the report demonstrated the need for a new investigation of a possible Saudi government role in the 2001 terror attacks.
9/11 report's classified '28 pages' about potential Saudi Arabia ties released
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The report – classified in December 2002 on orders of then president George W Bush – is almost certain to feed public suspicions that the Saudi government gave extensive support to Osama bin Laden before 9/11, and perhaps even directly to the 9/11 plotters themselves, as the US government looked the other way.Further info
https://www.theguardian.com/us-news/2016/jul/15/911-report-saudi-arabia-28-pages-releasedhttps://www.theguardian.com/us-news/2016/jul/16/911-report-release-saudi-arabia-us-relations