This answer relays observations and expert opinions related only to the narrow question of whether WTC 1, 2 and/or 7 were destroyed as part of a controlled demolition involving explosives.
This answer does not comment on the possibility of any conspiracy or involvement of any government nor any other such speculation.
Everything below is based on observed facts and credible expert opinions.
tldr; The use of the high explosive nanothermite to destroy the WTC towers on 9/11 is the theory most consistent with the evidence, and in particular it explains evidence that is inconsistent with the official theory.
In order to invalidate the above conclusion, one must refute or ignore all of the following:
Statements by numerous reputable witnesses, including experts appointed by the U.S. Government, indicated the presence of molten iron;
Reports concluding the presence of molten iron by numerous reputable agencies, including FEMA, a multi-national engineering firm commissioned by Deutsch Bank, numerous engineers, the NY Times;
The conclusion of Dr. Harrit in a peer reviewed journal article that inactive, unreacted nanothermite was present on the steel of WTC-7;
The presence of elemental sulfur;
The oxidized and sulfidized steel from WTC-7 reported in an appendix to a FEMA report;
The inability for kerosene (jet fuel) in general, and particularly in an open-air oxygen-starved environment, to reach temperatures hot enough to melt iron or steel;
The opinions of thousands of engineers and architects who have signed a petition indicating their belief that an explosive was involved in the destruction of the WTC towers.
NIST has never commented on whether the incendiary and high explosive called nanothermite was or could have been involved in the WTC towers collapse.
In other words, the use of nanothermite to destroy WTC 1, 2, and/or 7 is not inconsistent with the official report of the U.S. government. NIST, and by extension the U.S. government, has simply never commented on the possibility.
After reading the explanation below, if still interested in knowing more about the theory about the use of nanothermites in the WTC towers, you may wish to read The Mysterious Collapse of World Trade Center 7: Why the Final Official Report About 9/11 Is Unscientific and False by professor of philosophy, David Ray Griffin.
Thermodynamics, 2nd law
A key to the demolition theory is the presence of molten iron and/or steel. Molten iron and steel is inconsistent with the second law of thermodynamics namely, which roughly in this context translates as: the heat created around a chemical reaction cannot be hotter than the maximum temperature of that chemical reaction.
Since kerosene (jet fuel) burns at a max of 1030°C in open air, and steel melts at around 2700°C and iron at around 1500°C, kerosene cannot create molten iron or steel without violating the second law of thermodynamics.
Molten Iron and Steel: Witnesses
There are numerous eyewitness accounts of molten iron or steel. These include video of what appears to be molten metal dripping from the side of the WTC, as viewed in the linked YouTube video.
Dr. Abolhassan Astaneh, UC Berkley Professor of Structural Engineering and, according to Wikipedia, one of the leading structural engineers to investigate the collapse of the World Trade Center towers, said "I saw melting girders at World Trade Center".
Leslie Robertson, WTC Structural Engineer stated, "they pulled out the big block of concrete, and there was like a little river of steel, flowing ..."
Captain Philip Ruvolo, NYPD stated in front of others who were in agreement "You'd get down below, and you'd see molten steel. Molten steel running down the channel-ways. Like you were in a foundry. Like lava".
Ken Holden, director of NY Dept. of Design and Construction on C-SPAN 2, states "Underground it was still so hot that molten metal dripped down the sides of a wall".
Here is a photo of molten metal, from WABC-TV. The liquid flowing out of the buildings is identified by Gery Warner, PE and former engineer at Alcan Aluminum as molten iron.
The NY Times: A Search for Clues in Towers' Collapse
By JAMES GLANZ and ERIC LIPTON, Published: February 2, 2002:
Pieces of steel have also been found that were apparently melted and vaporized not solely because of the heat of fires, but also because of a corrosive contaminant that was somehow released in the conflagrations.
Perhaps the deepest mystery uncovered in the investigation involves extremely thin bits of steel collected from the trade towers and from 7 World Trade Center, a 47-story high rise that also collapsed for unknown reasons. The steel apparently melted away, but no fire in any of the buildings was believed to be hot enough to melt steel outright.
Another contemporaneous NY Times article (no longer available) stated "Steel members ... appear to have partially evaporated in extraordinarily high temperatures", and "Parts of the I-beam, once 5/8 inch thick, had vaporized."
Molten Iron and Steel: Physical Evidence
Credible reports indicating the presence of molten iron come from FEMA, a multinational engineering firm, numerous reputable engineers and professors who examined the physical evidence, and peer-reviewed journal articles.
There are numerous other credible sources, extensively documented in Griffin's book, starting at page 36.
In its report on WTC7 in May 2002, FEMA documents in Appendix C steel that has been melted and partially evaporated. That report states:
Evidence of a severe high temperature corrosion attack on the steel, including oxidation and sulfidation with subsequent intergranular melting, was readily visible in the near-surface microstructure.
The report “WTC Dust Signature Report: Composition and Morphology” prepared by multinational engineering firm RJ Lee Group for Deutsch Bank states:
Various metals (most notably iron and lead) were melted during the WTC Event, producing spherical metallic particles. Exposure of phases to high heat results in the formation of spherical particles due to surface tension. Figure 21 and Figure 22 show a spherical iron particle resulting from the melting of iron (or steel).
The presence of iron microparticles are a byproduct indicate the presence of molten iron. No other explanation has been brought forward for their presence (that I am aware of).
Presence of active, unreacted thermite
An undisputed, peer-reviewed paper entitled "Active Thermite Material Discovered in Dust from the 9/11 World Trace Center Catastrophe", by Niels H. Harrit et al., The Open Chemical Physics Journal, 2009, 2, 7-31, concludes:
Based on these observations, we conclude that the red layer of the red/gray chips we have discovered in the WTC dust is active, unreacted thermitic material, incorporating nanotechnology, and is a highly energetic pyrotechnic or explosive material.
There does not appear to be any plausible, competing theory that can explain the presence of nanothermites on the steel examined by Harrit.
Video documentary of experts
There is a video with a number of reputable experts, who comment on the possibility of the WTC tower destruction being a controlled demolition. The videos are here:
Experts Speak Out Videos
The videos posit that the WTC buildings fell by demolition, and addresses evidence that undermines the conclusions of the NIST report indicating that the buildings fell because of the airline collisions.
Expert opinions are presented by:
- Structural engineers (William Rice, PE; Richard Humenn, PE - Chief Electrical Design Engineer for WTC; David Topete, SE; Steven Dusterwald, SE)
- Mechanical Engineers (Anthony Szamboti, BSME; Gary Warner PE; Edward Munyak, PE)
- Chemical Engineers (David Gregg, PhD, CE; Jerry Lobdill, BSChE; Robert Kim Ireland, BSChE, US Army Corps.; Adam Parrott, BSChE; Mark Basile, BSChE; Robert Kim Ireland, BSChE, U.S. Army Corps of Engineers)
- Materials Science Engineers (Jeffrey Farrer, PhD)
- Metalurgists (Jason Cheshire, BSChE; Kathy McGrade, BS, ME)
- Explosives Engineers (Roland Angle, P.E., Green Beret)
- Professors (Lynn Margulis, PhD, National Medal of Science; Niels Harrit, PhD, Associate Professor Emeritus at U. of Copenhagen)
- Demolitions experts (Frank Cullinan, PE; Tom Sullivan)
- Architects (David Childs, architect of WTC-7; Leslie Young)
- Firefighters (Leslie Young)
- Fire safety experts (Edward Monyak, PE)
- Physicists (Robert Podolsky, MS; David Chandler, MS; Jonathan Barnett, Ph.D., Professor at WTI, Author of 2002 FEMA Appendix C Study on WTC 7 Steel; Niels Harrit, PhD Spectroscopy; Steven Jones, PhD)
- Other scientists (Kevin Ryan, BS Chemistry; Robert Bowman, Lt. Col USAF Ret., Former Director Advanced Space Programs; Jonathan Barnett, PhD, FEMA; William Woodward, PhD (Yale), MA (Princeton), BA (Harvard))
The experts involved comment on the inconsistencies between the NIST report and the reality of the physics under their expertise, and in particular how the fall of the twin towers and WTC-7 is not consistent with known physics. Most of their arguments are set out above.
Many other experts appear to be in agreement. This includes a petition by engineers and architects that has 2,408 signatures at the time of this writing. The petition includes the language:
We believe there is sufficient doubt about the official story and therefore the 9/11 investigation must be re-opened and must include a full inquiry into the possible use of explosives that might have been the actual cause of the destruction of the World Trade Center Twin Towers and Building 7.
ESO Theory and Evidence
For further interest, a web-site has put together a list of videos as part of a documentary discussing evidence of the demolition theory, which includes:
Rapid onset of destruction,
Constant acceleration at or near free-fall through what should have
been the path of greatest resistance,
Numerous eyewitness accounts of explosions including 118 FDNY
Lateral ejection of multi-ton steel framing members
distances of 600 feet at more than 60 mph,
Mid-air pulverization of 90,000 tons of concrete, and large volumes
of expanding pyroclastic-like dust clouds,
Isolated explosive ejections 20 to 60 stories below the “crush
Total destruction and dismemberment of all three buildings,
with 220 floors each an acre in size missing from the Twin Towers’
Several tons of molten steel/iron found in the debris piles,
Evidence of thermite incendiaries on steel beams,
Nanothermite composites and iron microspheres found in WTC dust
To be clear, and to repeat the disclaimer, there is no claim here regarding any conspiracy or involvement of the US government. It is tempting to make such a jump, but that would be purely speculative.
Concluding that the towers were destroyed by explosives does not necessarily mean the U.S. government participated or was complicit in such destruction.
The conclusion that nanothermites were involved in the destruction of the WTC towers is based on the observations of credible individuals, expert reports, and laws of physics.
The presence of nanothermites does not contradict the official NIST report and comments. According to Griffin's book (at p. 139), NIST has never commented on the possible use of nanothermites in WTC.
The NIST theory and explanation for the towers falling is inconsistent with evidence, namely the presence of molten iron and steel and the presence of nanothermites.
The use of nanothermites is the theory most consistent with the evidence to explain the fall of the WTC towers, particularly tower 7.