Check the design and materials used to weld the floor joist brackets to the outer structure. Check the debris photos to try to find any of those brackets still attached to the outer vertical beams.
And you have pics of that, or have checked them out?
You’re talking about the trusses. That’s what supported the floors. The supporting steel columns formed the outside perimeters of both buildings. The heat softened the trusses on the uppers floors and gravity took care of the rest.