And if they were looking for aluminum oxide and elemental iron, they would have found it from combusted airplane fuselage and melted building structural members. The diagonal shear fractures would be indifferent to the cause, since they happen only one way, if the steel loses its strength from being heated. This is a case of "experts" ignoring what was most likely to occur from the standpoint of combustion physics, and went in the direction of a preconceived notion.

This is my native territory. Aluminum powder is used prominently in large solid propellant motors. The aluminum fuel burns with a solid oxidizer (usually ammonium perchlorate). The aluminum in both rocket propellant and thermite has a thin oxide layer which prevents further oxidation until it is ignited. The combustion environment liquifies and evaporates the aluminum, which gets it free of the oxide layer and burns. A puddle of aluminum oxide slag may form within a rocket motor and is sometimes contained by a submerged exhaust nozzle.

There is no unique evidence for thermite. And, had thermite been used, the buildings would not have taken over an hour and a half to collapse.