In my opinion, based mainly on history, the security forces in the US were probably aware of some upcoming plot; it wouldn't be too surprising if they did know what the intended targets were and the means of carrying out the attack.
But to make the jump from knowing roughly of a plot doesn't mean they were actively involved. Why would the Republicans risk this? If it was true they would be out of office for half a century or more.
It's easy enough to get through some right-wing agenda in the US, it's been happening under the Democrats and Republicans for generations with few people noticing; Bush didn't need the attacks in order to carry out his agenda. In the past we see many cases of attacks be faked. The pretext for the German invasion of Poland was based on Polish forces invading Germany first; which of course was fake. The planned full-scale invasion of Cuba was going to be justified by the sinking of a US ship; which would of been fake. The invasion of Yugoslavia (under Clinton - not a Republican) was justified by genocide; which was also fake and so on and so on.
The US media buy any story and the US population swallow it; no matter how small it is. You don't need destruction on that scale to justify an attack on Afghanistan and Iraq and a clamping down of rights in the US. It's possible Bush knew something was coming and didn't act, or the security forces were instructed to carry on monitoring the involved rather than making the arrests. That may be an honest mistake or there may be something to it. I'm not sure which is the more likely.
They can always invent a story to push through an agenda, it might be a bit slower without people witnessing two symbols of America being brought down, but it would still of happened. Just like Iraq - which had nothing to do with the World Trade Center attacks was still invaded supported by huge majority in the US 90% and above.
Anyway, onto the story at hand.
Scientific analysis on WTC steel debris undertaken by BYU Professor Steven Jones proves that the twin towers were demolished by means of incendiary devices and the release of the conclusive evidence is imminent.
Steven Jones again, his bias should be enough to throw the evidence right into the bin. He's been claiming thermite has been used for months - with no evidence at all.
Now he claims to of found traces (on steel who's exact origins are largely unknown) of sulfur, which he claims is evidence that thermite was used to bring down the buildings. However in office buildings and aircraft you'll find a lot of sulfur and aluminum and lots of other things, so you really need traces of the other components of thermite, you should find 7 times as much barium is sulfur, yet his report does not mention any traces of barium.
Thermite itself is also a very bad way to demolish a building, it's actually used (or at least used to be used) to weld steel together because of its heat. You'd need masses and masses of thermite to even make a dent on a building like this. You'd really need something explosive to do the job, something like a large, fuel-laden aircraft. Oh wait that's what we've got.
Even more odd, thermite hasn't used sulfur as a binding agent for a decade or so, after this was pointed out to him he claims it was an exotic military thermite, which does use sulfur and which also explodes. Again no evidence to back up this claim.
I don't buy it. It's just the same as the Pentagon missile stuff... Sure if you ignore the hundreds of eye-witnesses who saw a large passenger aircraft and just put forward the few people who weren't sure what they saw you can make anything seem possible.
Steven Jones's work cannot be trusted at face value and should be scrutinized heavily.