The people who suspect an "inside job" for 9/11 are the ones you refer to as "truthers"---and there are quite a few in this category.
However, there are a few disturbing factors leading up to 9/11 that warrant in-depth investigation, because 9/11 was preventable. The plan of the lawless Bush/Cheney (Rumsfeld/Wolfowitz) administration was always to attack oil Iraq so that the ever-evil Dick Cheney could maneuver his way into grabbing a foothold in oil-rich Iraq that could then be turned into a long-desired opportunity to seize even more oil-rich territory---say, in Iran. The Bush/Cheney Treasury Secretary Paul O'Neill spoke of a time when he overheard Cheney and GW deep in conversation only four days after they had been sworn into office in January 2001---long before 9/11---plotting how to "come up with an excuse the American people would buy" to invade Iraq, a nation that never once attacked the U.S. (Ron Suskind, 2004: "The Price of Loyalty"). At least NINE warnings of a pending attack, both written and oral, were given to the lawless Bush/Cheney administration (Richard Clarke, 2004: "Against All Enemies"; C-SPAN's Senate testimony of Condoleeza Rice)...and these warnings were not just ignored, some of them inspired open hostility (NSA Director Richard Clarke, for example, was furiously scolded by GW for mentioning al-Qaeda and "pending attack" and was then was banned from Cabinet meetings!). What this suggests is that Cheney, Bush, Rumsfeld, and Wolfowitz wanted an attack---the "excuse the public would buy"---so that an ILLEGAL attack on Iraq could be sneaked in. The people who would die in this Cheney/GW-allowed attack were "collateral damage" in Cheney's evil means to a desired oil-grubbing end.
Facts the public does not know: Six weeks prior to the 9/11 attacks, the White House pressured the Chairman of the NY and NJ Port Authority Lewis M. Eisenberg, former Chairman of the Republican Finance Committee and big donor to the Cheney/Bush campaign, to award a lucrative 99-year lease to the World Trade Center to friends of the White House and of Rupert Murdoch, Larry M. Silverstein of Silverstein Properties and former Israeli commando Frank Lowy of Wesfield Properties for the LOWEST BID (only $3.2 billion, a tiny fraction of what this 99-year lease was actually worth). Eight hours after the planes hit the WTC, Silverstein (who owned nearby Tower 7) ordered his building (Tower 7) "pulled" (torn down) and then received $861 million from one of the many insurers for a building that had cost him $361 million. For the 13 insurers of the WTC (at least three of which were controlled by Bush family members Wirt Walker I, maternal uncle, Board Chairman; Wirt Walker III, maternal cousin; Marvin Bush, GW's brother; and CEO of the Texas insurer Jonathan Bush, paternal uncle), Silverstein demanded DOUBLE INDEMNITY (double payments) because "two planes hit the WTC"---and he got it. Perhaps irrelevant: Michael Eisenberg, Rupert Murdoch, and Larry Silverstein are senior members of the United Jewish Appeal, a group whose sole purpose is to raise money for Israel. Also interesting: Larry Flynt, in trying to help the Clintons, discovered that SECURACOM (changed to SECURATEC, Inc. post-9/11) was picked by the lawless Cheney/Bush administration to provide "security" for the WTC, for United Airlines (two 9/11 planes were United), and for Dulles International (two 9/11 planes flew out of Dulles). Guess which four Bush family members were Board members? Wirt I was Chairman, of course.
Riggs Bank laundered money for two of the hijackers through the wife of Prince Bandar, long-time friend of GW but also a trained Saudi military pilot and friend to the bin Laden family---and Wirt I was Chairman, Wirt I and Marvin were Board members, Jonathan Bush was bank president. I think once this criminal Bush/Cheney administration was aware of the approximate date of a pending attack, they made sure there was ample profits made. The 2003 ILLEGAL invasion of Iraq was pure evil.