Sifting through the evidence...

 When analyzing a crime scene you try to do as little speculation as possible. Where the evidence leaves off is where the theorizing begins and then only to the extent needed and keeping it as simple as possible since, as any programmer or planner knows that complexity breeds further complexity.

So, all we can say that we "know" is from what the officials have divulged.  It is the attempts to use this information to reconstruct the events, that we discover the information given us doesn't work. It does not result in what was observed because science gets in the way.

1. They gave us 19 skyjackers, but 8 were found alive and one died a year prior.
2. They gave us "calls from the planes" but the state of the technology at that time says that was not possible.
3. They gave us passengers boarding at specific gates at specific airports. But none of the cctv evidence that should be there exists.
4. They gave us untrained "pilots" to fly heavy aircraft at impossible speeds while performing maneuvers that experienced pilots say would over tax themselves.
5. They gave us impact videos that simply are not possible at all. Planes cannot crash into steel and concrete buildings without deforming at all, and pass through leaving momentary video evidence of no damage at all.

So these are the things that we know.  The simplest explanation for no. 5 is that it is manipulated video and not the "live" observations they told us we were looking at. 

So, we really don't know if there were actual planes, because if there were we did not see them.  The "eyewitnesses" are of no help, because they all saw and heard different things.  Things so different that there is nothing that confirms each others story.  This doesn't happen in real life.  If a car crashes into a truck, the witnesses may get the colors or timing wrong, but they will all say "a car and a truck".  We don't have that kind of baseline description here. The simplest way that could happen is, if there were no planes at all, leaving the witnesses to conjure with their own imaginations.

In fact, further analysis has discovered that some of the people who claimed to have seen the planes with their own eyes,  it turns out that they actually had no view of the planes at all. 

So, with no passengers, no hijackers, no pilots, no cell phone calls, no impossible speeds, no ace pilot maneuvers etc., the simplest answer to it all is that there were no planes.  In fact, if you go back to the initial coverage you'll note that the eyewitnesses didn't see planes but anchors viewing monitors in their studies saw them and tried to correct the eyewitnesses. Everyone looking at  tower 2 should have seen a plane!  They had no reason to lie because they had no clue as to what was happening.

Worse yet is, I'm only touching the most glaring points, in the background there are a lot more.  While Captain Scully's plane, flying low over the Hudson with it's engines cut off to boot, generated hundreds of 911 calls, Atta;s plane, flying low with it's engines open full bore, generated not one 911 call.