Building 7 had to be demolished by the perpetrators because if it had not been demolished investigations post 911 would have found it packed with nano-thermite for demolition.
Silverstein had to demolish WTC7 to destroy evidence.
Everything was pulverized. Concrete, steel office furniture, people, toilets and even the kitchen sink.
Wait, something did survive. The magic passport.
http://911research.wtc7.net/wtc/analysis/collapses/concrete.html"...Both reports of workers at Ground Zero and photographs of the area attest to the thoroughness of the pulverization of the concrete and other non-metallic solids in the towers. 3 An examination of our extensive archives of images of Ground Zero and its immediate surroundings reveals no recognizable objects such as slabs of concrete, glass, doors, or office furniture. The identifiable constituents of the rubble can be classified into just five categories:
pieces of steel from the towers' skeletons
pieces of aluminum cladding from the towers' exteriors
unrecognizable pieces of metal
pieces of paper
dust
Despite the presence of 400,000 cubic yards of concrete in each tower, the photographs reveal almost no evidence of macroscopic pieces of its remains..."