I am not naive enough to believe that the government has told us everything it knows about the events of 9/11.

For instance, I think we can be confident that a lot of connections between important Saudi families and the funding of al Qaeda’s attack have been hidden or glossed over.

But I am equally confident that there was no US government conspiracy to bring down any of the World Trade Center towers or to blow up the Pentagon.

It doesn’t require extraordinary trust to arrive at the conclusion that 3 planes crashed into buildings, bringing down towers 1 and 2 and damaging the Pentagon. There is lots of evidence that the planes hit the buildings, doing enormous structural damage and igniting fires that weakened key structural supports of the towers. The plane that hit the Pentagon was tracked by radar, and while the video evidence of the impact is scant, the plane didn’t just evaporate. It hit the building.

All the theories that rely on the melting point of steel are beside the point; nobody claims that steel melted in the first place. Rather, steel begins to lose strength above 600 degrees, and loses half its strength by 1100 degrees. The unique design of the WTC towers that made them essentially hollow boxes, creating lots of office space where normal skyscraper designs would not have, relied upon the outside of the building providing structural support. Rather than being a facade, those beams on the outside of the building that were broken by the planes were key structural elements.

The chief engineer of the Twin Towers–a man with a strong interest in defending the structural integrity of the building–can easily explain why it collapsed. He isn’t happy about doing so–who would be, especially since he was considered a genius for coming up with the engineering design that allowed the buildings to be so spacious? There is no mystery about why the towers collapsed.

The puzzle for most people was the collapse of WTC 7, and most conspiracy theories rest their case on the unexpected nature of its collapse hours after the two towers came down.

While the buildings were damaged by debris and were on fire for the entire day, no skyscraper had ever collapsed soley due to fire, and there was no indication that the debris strikes threatened the integrity of the building. Fires were raging out of control on a few floors, but fire alone had never taken down a skyscraper prior to 9/11. There were lots of theories about why the building collapsed, and it isn’t a surprise that some of them suggested that government action was involved.

It took many years for engineering analysts to come up with an explanation that wasn’t based at least partly on speculation. That analysis was done by the National Institutes of Standards and Technology (NIST).

“Our take-home message today is that the reason for the collapse of World Trade Center 7 is no longer a mystery,” NIST lead investigator Shyam Sunder told journalists at this morning’s press conference in Gaithersburg, Md. “WTC 7 collapsed because of fires fueled by office furnishings. It did not collapse from explosives or from diesel fuel fires.” [There had been speculation that a large tank of diesel fuel stored in the building contributed to the collapse.]

Conspiracy theorists have long pointed to the collapse of the 47-story structure as key evidence that the U.S. government orchestrated or abetted the 9/11 attacks. No planes struck the building, and the commonly available views of the exterior didn’t show significant damage. Yet, at 5:20 pm, 7 hours after the collapse of the Twin Towers (WTC 1 and 2), WTC 7 rapidly fell in on itself. Since WTC 7 housed Secret Service and CIA offices, conspiracy theorists claimed that the building was destroyed in a controlled demolition in order to obliterate evidence of the U.S. government’s complicity in the terrorist attacks. “It is impossible for a building to fall the way it fell without explosives being involved,” stated actress and TV personality Rosie O’Donnell of ABC’s The View in March 2007. “For the first time in history, steel was melted by fire. It is physically impossible,” she said.

Today’s report confirms that a fire was, indeed, the cause. “This is the first time that we are aware of, that a building taller than about 15 stories has collapsed primarily due to fires,”

One of the reasons why people have speculated that the collapse of the building was caused by explosives is that it looked like a controlled implosion–as did, superficially, the collapse of the Twin Towers. The similarity is mostly superficial though, as you can see if you have watched the videos of the towers collapsing.

Controlled implosions don’t start at the top of a building for obvious reasons–you want gravity to do most of the work so you weaken the structure from below causing the building to collapse in on itself, with down ideally being the only direction that debris goes. What happened in the North and South towers was different–the floors above the zones where the planes struck came down as a block, causing a cascade effect on all the subsequent floors. If the tops of the towers had been sheered off the rest of the towers would have remained standing, but once the top floors of the building gave way each floor below would suddenly have had to support the entire weight of everything above them, and they were never designed to do that.

WTC 7’s problem was different, given that it had a conventional design. Yet it, too, suffered from a progressive collapse. Why?

Over the course of 7 hours of fires the steel girders on several floors gradually expanded–thermal expansion–that put lateral pressure on concrete columns that had been designed for compressive verticle strength, not lateral strength.

After 7 hours of uncontrolled fires, a steel girder on Floor 13 lost its connection to one of the 81 columns supporting the building. Floor 13 collapsed, beginning a cascade of floor failures to Floor 5. Column 79, no longer supported by a girder, buckled, triggering a rapid succession of structural failures that moved from east to west. All 23 central columns, followed by the exterior columns, failed in what’s known as a “progressive collapse”–that is, local damage that spreads from one structural element to another, eventually resulting in the collapse of the entire structure.

NIST produced a short video explaining why the building collapsed. You can accept or not the objectivity of the analysis, but one key element leaves little doubt in my mind that explosives did not take down the building (other than the obvious fact that the windows did explode outward, that is): the first part of the exterior collapses was at the top of the building–the HVAC system, I believe. This is consistent with the explanation as outlined in the video and inconsistent with the theory that the building was brought down by controlled demolition.

The last element that suggests that there was no conspiracy to bring down these buildings is, in many ways, the most compelling. It simply would have required too many people doing too many things in too exposed a fashion for it to remain effectively hidden.

In contrast, while I have no reason to believe that JFK was assassinated by US government agents, the conspiracy to do so would not have been THAT difficult to hide. It wouldn’t require a large number of people acting in concert to achieve, and the logistics would be fairly easy. The weakest link would be Oswald himself, who was a loose cannon.

9/11 would require hundreds of people to work in concert over months. Keeping that secret would be a monumental task and likely beyond the capacity of any government or organization. And it is difficult to imagine that of those hundreds required not one of them would object or blurt out the truth.

In other words, Occam’s Razor leads us to the simplest conclusion: al Qaeda did it and unusual circumstances led to unpredictable but easily explained results.