Main towers On September 11, the North Tower (1 WTC) was hit by
American Airlines Flight 11 and the South Tower (2 WTC) was hit by
United Airlines Flight 175, both
Boeing 767 aircraft. The South Tower collapsed 56 minutes after the impact, and the North Tower collapsed 102 minutes after. An investigation by
NIST concluded that the collapse was caused by a combination of damage to support columns and fire insulation from the aircraft impacts and the weakening of columns and floors by
jet fuel ignited fires. Jones, among others, points to many descriptions by individuals working on the WTC rubble pile suggesting the presence of molten steel in the pile NIST found that the condition of the steel in the wreckage of the towers does not provide conclusive information on the condition of the building before the collapse and concluded that the material coming from the South Tower was molten
aluminum from the plane, which would have melted at lower temperatures than steel. NIST also 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. Jones and others have responded that they do not believe that thermite was used, but rather a form of thermite called
nano-thermite, a nanoenergetic material developed for military use, propellants, explosives, or pyrotechnics. Historically, explosive applications for traditional thermites have been limited by their relatively slow energy release rates. But because nano-thermites are created from reactant particles with proximities approaching the atomic scale, energy release rates are far improved. 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. A paper by
Zdeněk Bažant indicates that once collapse began, the kinetic energy imparted by a falling upper section onto the floor below was an order of magnitude greater than that which the lower section could support. Controlled demolition of a building to
code 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. Controlled demolition is traditionally done from the bottom of buildings rather than the top, although there are exceptions depending on structural design. There is little dispute that the collapse started high up at the point where the aircraft struck. Furthermore, any explosives would have to withstand the impact of the airliners. of flashes and loud explosions immediately before the fall. Eyewitnesses have repeatedly reported of explosions happening before the collapse of the WTC towers, and the organization "International Center for 9/11 Studies" has published videos obtained from NIST, together with indications about when such explosions could be heard. There are many types of loud sharp noises that are not caused by explosives, and
seismographic records of the collapse do not show evidence of explosions. Jones and others have argued that horizontal puffs of smoke seen during the collapse of the towers would indicate that the towers had been brought down by controlled explosions. NIST attributes these puffs to air pressure, created by the decreasing volume of the falling building above, traveling down elevator shafts and exiting from the open elevator shaft doors on lower levels. In September 2011, Iranian president
Mahmoud Ahmadinejad, who holds a PhD in Transportation Engineering and Planning, said that it would have been impossible for two jetliners to bring down the towers simply by hitting them and that some kind of planned explosion must have taken place. Al-Qaeda sharply criticized Ahmadinejad in their English-language publication,
Inspire, calling his assertions "a ridiculous belief that stands in the face of all logic and evidence".
7 World Trade Center Proponents of World Trade Center controlled demolition theories allege that
7 World Trade Center—a 47-story skyscraper that stood across Vesey Street north of the main part of the
World Trade Center site—was intentionally destroyed with explosives. Unlike the Twin Towers, 7 World Trade Center was not hit by a plane, although it was hit by debris from the Twin Towers and was damaged by fires which burned for seven hours, until it collapsed completely at about 5:20 p.m. on the evening of September 11 (a new building has been erected on the site of the old and opened in May 2006). Several videos of the collapse event exist in the public domain, thus enabling comparative analysis from different angles of perspective. Proponents typically say the collapse of 7 World Trade Center was not mentioned in the
9/11 Commission Report and that the federal body charged with investigating the event,
NIST, required seven years to conduct its investigation and issue a report. In November 2010,
Fox News reporter
Geraldo Rivera hosted members of a television ad campaign called "BuildingWhat?", a series of commercials in which 9/11 family members ask questions about 7 World Trade Center and call for an investigation into its collapse. Rivera called the television ads "not so easy to dismiss as those demonstrators were," and stated that, "If explosives were involved, that would mean the most obnoxious protesters in recent years ... were right." Days later, Rivera appeared on the program
Freedom Watch with legal analyst Judge
Andrew Napolitano on the
Fox Business Network to discuss the BuildingWhat? TV ad campaign. Napolitano stated, "It's hard for me to believe that [7 World Trade Center] came down by itself. I was gratified to see Geraldo Rivera investigating it." Some proponents of World Trade Center controlled demolition theories suggest that 7 WTC was demolished because it may have served as an operational center for the demolition of the Twin Towers, while others suggest that government insiders may have wanted to destroy key files held in the building pertaining to corporate fraud. The WTC buildings housed dozens of federal, state and local government agencies. According to a statement reported by the
BBC,
Loose Change film producer
Dylan Avery thinks the destruction of the building was suspicious because it housed some unusual tenants, including a clandestine CIA office on the 25th floor, an outpost of the U.S. Secret Service, the Securities and Exchange Commission, and New York City's emergency command center. No other
steel frame high rise had ever before collapsed because of a fire, although there have been previous cases of collapses or partial collapses of smaller steel buildings due to fire. However, the ability of such a building to be completely destroyed by fire would be demonstrated by the collapse of the
Plasco Building in
Tehran in 2017 and the
Wilton Paes de Almeida Building in
São Paulo,
Brazil, the following year. In addition, NIST claims debris ejected during the collapse of 1 WTC caused significant structural damage in 7 WTC before the fire.
BBC News reported the collapse of 7 WTC twenty minutes before it actually fell. The BBC has stated that many news sources were reporting the imminent collapse of 7 WTC on the day of the attacks. Jane Standley, 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. In the PBS documentary
America Rebuilds, which aired in September 2002,
Larry Silverstein, the owner of 7 WTC 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." Silverstein issued a statement that it was the firefighting team, not the building, that was to be pulled, contradicting theorists' allegation that "pull" was used in a demolition-related sense. A draft version of its final report on the collapse of 7 WTC was released in August 2008. The agency has attributed the slowness of this investigation to the complexity of the computer model it used, which simulated the collapse from the moment it begins all the way to the ground; and NIST says the time taken on the investigation into 7 WTC is comparable to the time taken to investigate an aircraft crash. NIST released its final report on the collapse of 7 World Trade Center on November 20, 2008. Investigators used videos, photographs and building design documents to come to their conclusions. The investigation could not include physical evidence as the materials from the building lacked characteristics allowing them to be positively identified and were therefore disposed of prior to the initiation of the investigation. The report concluded that the building's collapse was 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 building 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 7 WTC was brought down with explosives and concluded that a blast event did not occur, that the "use of thermite ... to sever columns in 7 WTC on 9/11/01 was unlikely". The conclusions of their report and the intended outcome of the computational model were already decided in 2015. Some other researchers have criticized their conclusions. These conclusions were present in Hulsey's group final report: ==Criticism==