While many of the theories are bizarre, one longstanding view is the towers would not have collapsed in the way they did if they were hit by aircraft.
Some claim there must have been a “controlled detonation” at ground level for the Twin Towers to fall in on themselves as they did.
One key part of their argument is the collapse of a third smaller tower, called Building 7, at the World Trade Centre complex, several hours after the huge skyscrapers fell.
Until now, the theory has been just that and confined to the online forums of conspiracy theory websites.
But now, the University of Alaska is sponsoring a full investigation into claims that World Trade Center Building 7 was brought down by a controlled demolition during the 9/11 attacks.
The official version of events is that fire spread to Building 7, from the main towers, devastating the structure, and causing it also to fall in on itself.
Footage of the tower consumed by fire emerged in 2011, and it was thought the conspiracy may have been killed off.
But Dr J Leroy Husley, chair of the University of Alaska Fairbanks’ (UAF) Civil and Environmental Engineering Department, has partnered with architects and engineers linked to campaign group 9/11 Truth to evaluate the causes of its collapse.
A report on Activistpost.com said: “Although questions still remain about how the two planes that hit the Twin Towers could cause the total collapse of the high-rise buildings, many 9/11 researchers now focus on the mysterious collapse of Building 7.
“A number of 9/11 family members point to the collapse of WTC7 as a possible crack in the official story that could spark a new national conversation on the events of that day.
“WTC7 was not hit by a plane that day; however, it collapsed at 5:20 p.m. according to the National Institute of Standards and Technology (NIST), the official cause for the collapse was office fires.
“A growing number of family members, activists, architects and engineers question the official theory for collapse and are seeking a new investigation into WTC7.”
Dr Hulsey and the WTC7 Evaluation project hope to answer some of the remaining questions.
The project is a two-year study that is being crowd-funded through Architects & Engineers for 9/11 Truth.
Dr Hulsey has invited professionals from the fields of structural engineering, civil engineering, mechanical engineering, architecture, building design and construction, physics, maths, and science, as well as everyday citizens, to get involved.
He said: “Over the next year, with a team of PhD students, I will be rebuilding World Trade Center building 7, using the same drawings that were used to build it originally we will reconstruct it digitally.
“NIST says the building fell down due to office fires. Our investigation will evaluate the probability that this was the cause of the collapse.”
The researchers have promised a “completely open and transparent investigation into the cause of World Trade Center Building 7’s collapse”, and will post every step of their scientific process onWTC7Evaluation.org.
The WTC7 Evaluation project will also include a review by a committee of technical experts who will vet the research being conducted by Dr Hulsey and his students.
The professor and his team are soliciting financial support from the public from now until the completion of the study in April 2017.
Ted Walter, Director of Strategy and Development for A&E 9/11 Truth, is in charge of working with the professor and raising money to fund the WTC7 Evaluation.
He said: “They are coming up with different scenarios of how hot the fires could have been in different parts of the building, and then for the next six months they will be running tests and scenarios.
“The last few months, early next year, will be all about putting the findings into a final report.”
The team want the final report published in peer-reviewed engineering journals.
Mr Walter added: “We hope to gain significant traction in the engineering community by providing an authoritative refutation of NIST’s report, by showing that there is no way that fires could have brought down building 7.”
He also wants the report “given to every member of Congress”.