Substance?
Certainly not. Unlike the other posters here, I have not been directly involved in the investigation, and neither do I have any relevant professional qualifications.
Unusually, perhaps, I'm just an interested bystander; an ordinary Joe if you like. Put me down as the one person in this thread who doesn't have an axe to grind or an inside track. As such, I speak as I find. And, reading the (admittedly biassed, liberal and probably treacherous) British media, along with the various analyses by people who would seem from their experience and qualifications to know what they're talking about, I have reached the conclusions I have.
All I can say as an 'expert witness' is that I have seen plenty of buildings of various sizes and construction made to collapse by various means, and I have no reason to doubt that the collapse of the Twin Towers and the damage to the surrounding buildings was caused by aircraft impact.