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.
No, I am talking about the brackets (not even welded all the way around) on the outside vertical beams that provided vertical support for the outside ends of the joists/trusses.
Here's a site I hadn't seen before that seems to have plenty of pics/photos/theories.
http://www.sharpprintinginc.com/911/index.php?module=pagemaster&PAGE_user_op=view_page&PAGE_id=161&MMN_position=154:154
There is apparently a case to be made for poor joint design of the core columns (butt welds, all on the same horizontal plane - right where fractures of the columns occurred).