The most detailed statements of the hypothesis have come from physicist Steven Jones, architect Richard Gage, software engineer Jim Hoffman, theologian David Ray Griffin, and author Webster Griffin Tarpley. Proponents argue that the aircraft impacts and resulting fires could not have weakened the buildings sufficiently to initiate collapse and that the buildings would in any case not have collapsed as completely, and quickly as they did without an additional source of destructive energy to undermine their structure. Various sources of this energy have been proposed; the use of thermite, explosives, or some combination thereof is the most common suggestion being made today. The National Institute of Standards and Technology (NIST), an agency of the United States Department of Commerce, has rejected the hypothesis that collapse due to fire would be impossible, as has the engineering community. Members of the 9/11 Truth movement have filed Requests for Correction to the NIST report,. Only one of their requests resulted in a change to correct an inconsistency between two parts of the NIST report. This resulted with another appeal from the 9/11 Truth movement.
Engineers were in fact initially surprised by the collapses and at least one considered explosives as a possible explanation. The broad outlines of an explanation that did not involve such explosives quickly emerged, however, and took its current shape in the 2005 NIST report. It has come to be known as "the official account" among proponents of controlled demolition.
The hypothesis was first suggested in October 2001. An early book-length treatment of the hypothesis inspired both David Ray Griffin's critical inquiry as well as the Popular Mechanics investigation of 9/11 conspiracy theories. In late 2005, Brigham Young University Professor of Physics Steven Jones made his own pursuit of the hypothesis public. Even before publication of the article in 2006, his interest in the hypothesis brought a measure of media exposure to the theory. BYU responded to Jones' "increasingly speculative and accusatory" statements by placing him on paid leave in September, 2006. Shortly thereafter, Jones accepted BYU's offer of early retirement.
Proponents of the controlled demolition have questioned the "pancake collapse" hypothesis originally suggested by FEMA which the NIST also rejected and finally replaced with the current column failure theory.
In its final report, NIST stated that it "found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001" and posted a FAQ about related issues to its website in August of 2006. The major elements of the hypothesis have been rebutted in mainstream engineering scholarship, where its proponents are considered "outsiders".
A 2006 poll found that 6 percent of Americans considered it "very likely" that "the collapse of the twin towers in New York was aided by explosives secretly planted in the two buildings", while another 10% found it "somewhat likely". 77% found the demolition hypothesis "unlikely". A 2007 poll found that 67% of Americans fault the 9/11 Commission for not investigating the collapse of World Trade Center 7. An August 2007 Zogby poll found that 4.8% of Americans believe that "certain US government elements actively planned or assisted some aspects of the attacks".
In June 2008, Arizona State Senator Karen Johnson delivered a letter to the office of U.S. Senator John McCain asking him to meet with a group of professionals to discuss the events of 9/11. She also gave a speech on the floor of the Arizona Senate that included her support for the demolition theory, its proponents, and its relevance to current foreign policy in the US. Johnson said in her speech:
"You don’t have to embrace every theory about 9/11. Indeed, there are some that should be soundly rejected. But if you believe, as these scientists, architects and engineers do, that the buildings were brought down by explosive demolition, then you must also agree that we need a new investigation. I have no preconceived notions about who did it and I am not pointing the finger of blame at anyone. But I do think that the worst attack on U.S. soil in American history deserves the best investigation possible."
7 World Trade Center was a 47-story steel-framed skyscraper that stood across Vesey Street north of the main WTC complex. Its tenants included Salomon Smith Barney (which leased 44% of the available office space), ITT Hartford Insurance Group (8%), the Securities and Exchange Commission (8%), the Secret Service (5%), and the Internal Revenue Service (IRS) Regional Council (3%). Smaller tenants included the Central Intelligence Agency (CIA) and the Department of Defense, though these shared a single floor with the IRS. Altogether, U.S. local, state, and federal government agencies occupied 11 of the overall 47 floors, or about 28% of the available 39 floors of office space. Though damaged by fires which burned for seven hours, it was not hit by a plane, and collapsed at about 5:20 p.m. EDT on the evening of September 11, 2001.
Some journalists commenting on the nature of the collapse of WTC 7 said that it resembled a controlled demolition, although the explanation that fires in the building, started by falling debris from the collapse of WTC 1, had caused the structure to fail, quickly emerged. No steel-frame high rise had ever before collapsed because of a fire. BBC News reported the collapse of WTC 7 twenty minutes before it actually fell. The BBC has stated that many news sources were reporting the imminent collapse of WTC 7 on the day of the attacks.. Jane Staley, the reporter who announced the collapse prematurely, called it a "very small and very honest mistake" caused by her thinking on her feet after being confronted with a report she had no way of checking.
Proponents of controlled demolition often emphasize the collapse of WTC 7 because its collapse looked like a bottom-to-top conventional controlled demolition, as opposed to the more explosive top-to-bottom collapses of the two main towers. Support for this theory comes from features argued to have been visually observed in the collapse--the speed of the collapse, the way it fell down vertically and symmetrically, the rapid onset, and the way the center of the roof fell first. Steven Jones claims that the presence of sulphur is evidence that indicates the use of explosives such as thermate, along with reports of molten metal and extremely high temperatures in the rubble. However, metallurgist Prof. Richard Sisson asserts that the sulfur came from gypsum in the wallboards, an opinion which was also given in the NIST report.
In the PBS documentary America Rebuilds, which aired in September 2002, Larry Silverstein, the owner of WTC 7 and leaseholder and insurance policy holder for the remainder of the WTC Complex, recalled a discussion with the fire department in which doubts about containing the fires were expressed. Silverstein recalled saying, "We've had such terrible loss of life, maybe the smartest thing to do is pull it". "They made that decision to pull", he recalled, "and we watched the building collapse." Some proponents of the controlled demolition hypothesis have taken the remark as a confession that he ordered the building to be demolished. Silverstein issued a statement that rejects this interpretation, asserting that it was the firefighting team, not the building, that was to be pulled.
Hugo Bachmann and Jörg Schneider, professors emeritus of structural engineering at the Swiss Federal Institute of Technology, believe that WTC 7 was intentionally demolished based on video footage. The 9/11 Family Steering Committee also asked what happened to WTC 7 in their 'Questions Regarding the 9/11 Commission Interview of Mayor Rudy Giuliani,' asking, "On 9/11, no aircraft hit WTC 7. Why did the building fall at 5:20 PM that evening?"
In 2002 the National Institute of Standards and Technology began a general investigation into the collapse of the World Trade Center but soon made a decision to focus on the collapse of the Twin Towers first. A draft version of its final report on the collapse of building 7 was released in August 2008. The agency has blamed the slowness of this investigation on the complexity of the computer model it is using, which simulates the collapse from the moment it begins all the way to the ground, and notes that the time taken on the investigation into building 7 is comparable to the time taken to investigate an aircraft crash. The agency also notes another 80 boxes of documents related to WTC7 were also found and had to be analyzed. These delays fueled suspicion that the agency was struggling to come up with a plausible conclusion.
Preliminary investigations did not include the mechanics of the actual collapse, concentrating instead on the events leading up to it. However, the final draft report on the collapse of WTC7 by NIST provides a detailed investigation into the collapse timeline, starting with the failure of a critical column, Column 79 (initial failure event). 6 seconds later, the collapse of the East Penthouse on the roof was visible. The collapse of the core columns progressed from east to west for another 6.9 seconds (12.9 seconds total since the initial failure event). At this point, the report says, "all the interior columns had buckled" and "the remaining exterior structure above began to fall vertically as a single unit." To calculate the timeline of the collapse of the rest of the building, NIST focused on the time between the initial collapse of the roofline and the last position that the complete roofline could be observed before portions of it started to become obscured by dust, at the top of Floor 29. NIST calculated the timeline for this observable descent as 5.4 seconds and calculated the theoretical free-fall time for the same portion of the building as 3.9 seconds, and concluded that, "The actual collapse time of the upper 18 floors of the north face of WTC7 (the floors clearly visible in the video evidence) was 40 percent greater than the computed free fall time. This was consistent with physical principles.
Following a three year investigation NIST released a draft version of its final report on the collapse on August 21, 2008. Investigators used videos, photographs and building design documents to come to their conclusions. The report concluded that the building collapsed due to the effects of the fires which burned for almost seven hours. The fatal blow to the building came when the 13th floor collapsed, weakening a critical steel support column that led to catastrophic failure, and extreme heat caused some steel beams to lose strength, causing further failures throughout the buildings until the entire structure succumbed. Also cited as a factor was the collapse of the nearby towers, which broke the city water main, leaving the sprinkler system in the bottom half of the building without water.
NIST considered the possibility that the towers were brought down with explosives and concluded that a blast event did not occur. The investigation noted that no blast was audible on recordings of the collapse and that no blast was reported by witnesses, even though it would have been audible at a level of at least 130-140 decibels at a distance of half a mile. NIST also investigated the possibility that the collapse was caused by thermite and concluded that it is unlikely that the quantities needed could have been carried into the building undetected. The theory that fires from the large amount of diesel fuel stored in the building caused the collapse was also investigated and ruled out.
World Trade Center developer Larry Silverstein said, "Hopefully this thorough report puts to rest the various 9/11 conspiracy theories, which dishonor the men and women who lost their lives on that terrible day." Richard Gage, leader of the group Architects and Engineers for 9/11 Truth said, "How much longer do we have to endure the coverup of how Building 7 was destroyed?". James Quintiere, professor of fire protection engineering at the University of Maryland, College Park, who does not believe explosives brought down the towers, questioned how the agency came to its conclusions, remarking, "They don't have the expertise on explosives. Following the NIST final draft on Building 7 in August of 2008, a group of demolition proponents submitted a response challenging several points of the draft.
The hypothesis suggests that the fires could not have been hot enough to weaken the steel structure of the two towers to the point of collapse. However, those who pursue the hypothesis emphasize evidence of temperatures well beyond those that, by general consensus, can be attributed to the fires. Molten metal streamed out of the south tower shortly before it collapsed. There are first-hand reports of claims that molten metal was discovered in the piles of rubble in the weeks following the collapses. Steven Jones has argued that the molten metal may have been iron, a byproduct of a thermite reaction. Thermite reactions can reach temperatures of up to 4500°F (2500°C), well beyond the temperature (approximately 1500°C (2732°F) required to melt structural steel.
In response, NIST has pointed out that cutting through the vertical columns would require planting an enormous amount of explosives inconspicuously in highly secured buildings, then igniting it remotely while keeping it in contact with the columns. NIST concluded that some of the observed molten metal may have been molten aluminum from the fuselage of the plane. Aluminum melts at significantly lower temperatures than steel.
The controlled demolition hypothesis is also offered to explain a belief that the towers collapsed close to free fall speed. Most estimates agree that the structures offered little resistance to the progress of the collapses and that they took about 50% longer than a free falling object dropped from the tops of the towers. Without explosives to destroy the internal support structure of the WTC towers, argue proponents of controlled demolition, the fall of the towers would violate the principle of conservation of momentum. Others say that these claims are only supported by intuition without any quantitative analysis. They point to their own analyses posted on a website suggesting that the fall may be explained without violating the principle of conservation of momentum and without requiring any explosives.
The NIST report provides an analysis of the structural response of the building only up to the point where collapse begins, and asserts that the enormous kinetic energy transferred by the falling part of the building makes "progressive collapse" inevitable once an initial collapse occurs. Supporters of the controlled demolition hypothesis often emphasize that NIST did not simulate the structural response of the lower parts of the buildings, which they find of primary interest, but do not analyze either. A paper by Zdeněk Bažant indicates that once collapse began, the kinetic energy imparted by a falling upper section onto the floor below was at least ten times greater than that which the lower section could support.
Engineers who have investigated the collapses generally deny that controlled demolition is required to understand the structural response of the buildings. While the top of one of the towers did tilt significantly, it could not ultimately have fallen into the street, they argue, because any such tilting would place sufficient stress on the lower story (acting as a pivot) that it would collapse long before the top had sufficiently shifted its center of gravity. Indeed, they argue, there is very little difference between progressive collapse with or without explosives in terms of the resistance that the structures could provide after collapse began. Controlled demolition of a building requires weeks of preparation, including laying large quantities of explosive and cutting through beams, which would have rendered the building highly dangerous and which would have to be done without attracting the attention of the thousands of people who worked in the building.
Several other common points underlie the Controlled Demolition hypothesis. First, proponents underscore similarities in video footage of the collapses of the WTC towers with footage of known controlled demolitions. One of the most commonly cited similarities are the tightly focused horizontal plumes of smoke and debris being ejected from the twin towers just before and during the collapse. While these plumes are attributed by demolition theory opponents to material ejected due to the compression of air as the floors collapsed, proponents claim that they may be evidence for exploding demolition charges ("squibs").
Proponents claim that eyewitness accounts made by firefighters and emergency medical responders of explosions just prior to the start of the collapse of the towers are suggestive of controlled demolition. However there are many causes of loud sharp noises that are not caused by explosives, and seismographic records of the collapse do not show evidence of explosions.
Additionally, the production and expansion of the enormous dust clouds that covered Manhattan after the collapses has also been taken as an indication of an additional source of energy, such as explosives. Some proponents suggest that the energy required for this expansion alone (ignoring the energy needed to slice the steel and pulverize the concrete and other materials) exceeded the gravitational energy available by 9.7 × 1012 J to 4.2 × 1013 J. This corresponds to extra energy of about 2000 to 10000 tons of TNT, or 40 to 200 times the yield of the most powerful conventional bomb. NIST attributes these clouds to the ejection of air from compressed parts of the building.
The debris removal process began shortly after the attacks, and concluded in May 2002. Some members of the 9/11 Truth Movement allege that engineers were not granted access to Ground Zero until most of the debris had been removed, while others allege that engineers were not granted access to Ground Zero or even the salvage yards at all. However, Robert F. Shea of FEMA testified to the House of Representatives that, "Because of the importance of the rescue effort at the World Trade Center complex, it was clear that information would have to be gathered without interfering with response and rescue activities. Based on this fact, the FEMA-ASCE team first visited the site on October 6,  but gathered information from others who had been on-site before this date." Regarding access to the scrap yards, appendix D of the FEMA report states that, "As of March 15, 2002, a total of 131 engineer visits had been made to these yards on 57 separate days.
A call to action by Bill Manning, the chief editor of the trade journal Fire Engineering, is often quoted in this connection. Manning called the early ASCE investigation (which would later turn into the FEMA building performance study) a "half-baked farce" and said that "the destruction and removal of evidence must stop immediately." He said that the cleanup of the WTC site differed in many respects from that of other engineering disasters. In defense of the decision to dispose of the steel, Mayor Bloomberg said: "If you want to take a look at the construction methods and the design, that's in this day and age what computers do." David Ray Griffin notes that this is exactly what Manning had worried about when he warned that "the investigation into the World Trade Center fire and collapse will amount to paper-and computer-generated hypotheticals."
However, allegations against a "speedy removal" of the steel hampering the engineering investigations appear to be unfounded, according to Dr. Gene Corley, head of the BPAT team and one of the lead engineers for the investigation by the National Institute of Standards and Technology (NIST), which began in September 2002. He testified to the House of Representatives in March 2002 that, "There has been some concern expressed by others that the work of the team has been hampered because debris was removed from the site and has subsequently been processed for recycling. This is not the case. The team has had full access to the scrap yards and to the site and has been able to obtain numerous samples. At this point there is no indication that having access to each piece of steel from the World Trade Center would make a significant difference to understanding the performance of the structures".
On February 28, 2007, NIST's conclusion that it "found no corroborating evidence for alternative hypotheses suggesting that the WTC towers were brought down by controlled demolition using explosives planted prior to September 11, 2001" was challenged in a request for correction (per Section 515 of Public Law 106-554). The request was based on NIST's acknowledgement that it did not investigate controlled demolition nor look for evidence.
Thomas Eagar, a professor of materials science and engineering at the Massachusetts Institute of Technology, also dismissed the controlled demolition hypothesis. Eagar remarked, "These people (in the 9/11 truth movement) use the 'reverse scientific method.' They determine what happened, throw out all the data that doesn't fit their conclusion, and then hail their findings as the only possible conclusion. Finally, Leslie Robertson, who helped design the Twin Towers, debated the issue with Steven Jones on a radio program in December 2006.
In April 2008, a letter by advocates of the demolition hypothesis was published as a paper in an online civil engineering journal.. In July 2008, an article was published in an environmental journal.
The demolition hypothesis first entered mainstream media by way of negative press coverage of "9/11 conspiracy theories" or "9/11 myths". Critical articles in Popular Mechanics, which were later expanded into a book, and the popular magazine Skeptic presented rebuttals to the hypothesis for a mainstream audience. In 2006, a New York Magazine reported that, "A new generation of conspiracy theorists is at work on a secret history of New York’s most terrible day." The hypothesis has been cited by numerous popular actors, musicians and politicians, including Charlie Sheen, Martin Sheen, Willie Nelson, Mos Def, and former Governor of Minnesota, Jesse Ventura. The hypothesis also features prominently in the controversial online documentary Zeitgeist, the Movie.