Such "red flags," as they are sometimes called, were the meat and potatoes of the keynote speech on Friday night by Alex Jones, who is the William Jennings Bryan of the 9/11 band. Mr. Jones, a syndicated radio host, is known for his larynx-tearing screeds against corruption — fiery, almost preacherly, addresses in which he sweats, balls his fists and often swerves from quoting Roman history to using foul language in a single breath.

At the lectern Friday night, beside a digital projection reading "History of Government Sponsored Terrorism," Mr. Jones set forth the central tenets of 9/11 Truth: that the military command that monitors aircraft "stood down" on the day of the attacks; that President Bush addressed children in a Florida classroom instead of being whisked off to the White House; that the hijackers, despite what the authorities say, were trained at American military bases; and that the towers did not collapse because of burning fuel and weakened steel but because of a "controlled demolition" caused by pre-set bombs.

According to the group's Web site, the motive for faking a terrorist attack was to allow the administration "to instantly implement policies its members have long supported, but which were otherwise infeasible."

The controlled-demolition theory is the sine qua non of the 9/11 movement — its basic claim and, in some sense, the one upon which all others rest. It is, of course, directly contradicted by the 10,000-page investigation by the National Institute of Standards and Technology, which held that jet-fuel fires distressed the towers' structure, which eventually collapsed.

The movement's answer to that report was written by Steven E. Jones, a professor of physics at Brigham Young University and the movement's expert in the matter of collapse. Dr. Jones, unlike Alex Jones, is a soft-spoken man who lets his writing do the talking. He composed an account of the destruction of the towers (www.physics.byu.edu/research/energy/htm7.html) that holds that "pre-positioned cutter-charges" brought the buildings down.

Image David Kubiak, one of the speakers at the International Education and Strategy Conference for 9/11 Truth. Credit... Joe Tabacca for The New York Times

Like a prior generation of skeptics — those who doubted, say, the Warren Commission or the government's account of the Gulf of Tonkin attack — the 9/11 Truthers are dogged, at home and in the office, by friends and family who suspect that they may, in fact, be completely nuts.