The steel from the debris of the World Trade Centers provides evidence that the buildings were demolished.
Specifically, metal tests on steel remains from the world trade center show something never before seen in building fires: reactions which caused "intergranular melting capable of turning a solid steel girder into Swiss cheese." Sulfur and other elements commonly used in high-explosives were also found. The New York Times described this as "perhaps the deepest mystery uncovered in the investigation." (this quote is contained in the following link)
As stated by the scientists who studied this phenomenon:
"A one-inch column has been reduced to half-inch thickness. Its edges -- which are curled like a paper scroll -- have been thinned to almost razor sharpness. Gaping holes -- some larger than a silver dollar -- let light shine through a formerly solid steel flange. This Swiss cheese appearance shocked all of the fire-wise professors, who expected to see distortion and bending -- but not holes"
Moreover, an expert stated about World Trade Center building 7 (the third building in the World Trade Center to collapse on 9/11):
"A combination of an uncontrolled fire and the structural damage might have been able to bring the building down, some engineers said. But that would not explain steel members in the debris pile that appear to have been PARTLY EVAPORATED in extraordinarily high temperatures" (pay-per-view); or see a cached version of the article here.
Steel does not evaporate unless it is heated to at least 5000 degrees Fahrenheit. Everyone agrees that fires from conventional building fires are thousands of degrees cooler than that.