It was carried out by Israel and Zionist factions of the American public.
https://en.wikipedia.org/wiki/Project_for_the_New_American_Century Project for a New American Century was 25 Zionist Neocons, 10 of which went on to form the Bush administration that began the Zionist wars in the Middle East, and continue to warmonger in Syria, such as Donald Rumsfeld and Dick Cheny, who's oil company Genie Oil was granted the Oil rights of the Israeli occupied Golan Heights, rightfully a part of Syria. Genie Oil board of executives also include Jacob Rothschild, whos family founded Israel and owns the world's money, Rupert Murdoch chairman of Fox News, and James Woolsey ex director of the CIA. These are the people that benefitted from the Zionist propaganda of the War on Terror.
The Narrative which rolled out after 9/11 was ready within the following days, and only three weeks later there were American boots on the ground in Afghanistan.
The only people arrested in connection with the 9/11 attacks were Israeli citizens. 5 were arrested after being seen dancing, celebrating, and cheering when the planes hit the towers. They were questioned, released, and later said on Israeli TV that they were sent by the Mossad to document the events.
Soon after 9/11 the largest expulsion of spies in US history occurred when 200 Israeli spies posing as art students were expelled, who were "selling art" door to door, or office to office, in American government facilities, and 14 who were doing an "art project" in the World Trade Center, which included placing what appears to be explosives along the walls of a room, in the weeks before the attack. I'm going off memory here but you can read about it here:
https://www.bibliotecapleyades.net/sociopolitica/esp_sociopol_911_125.htm
"Mossad's former motto, be-tachbūlōt ta`aseh lekhā milchāmāh (Hebrew: בתחבולות תעשה לך מלחמה) is a quote from the Bible (Proverbs 24:6): "For by wise guidance you can wage your war" (NRSV). The motto was later changed to another Proverbs passage: be-'éyn tachbūlōt yippol `ām; ū-teshū`āh be-rov yō'éts (Hebrew: באין תחבולות יפול עם, ותשועה ברוב יועץ, Proverbs 11:14). This is translated by NRSV as: "Where there is no guidance, a nation falls, but in an abundance of counselors there is safety." Put these Hebrew phrases through Google translate. I took this paragraph from Wikipedia. Here's what you get -
Deceit is essential in warfare. This is what is written on the Mossad flag. Now it is changed to -
In the absence of tricks, a people will fall, and salvation in the majority of advisers. Put it through Google translate and see for yourself.
Christopher Bollyn began writing the book "Solving 9/11" in the early 2000's, gathering information and writing it for over 10 years, and realized through that time the Zionist hand in 9/11. His research is well founded, well expounded, and anyone who honestly listens to it can understand these world events in a comprehensive and clear manner.