I was away this weekend visiting with my dad and brought up the discussion occurring on this site about 9/11 conspiracies. He directed me to this series from PBS\NOVA. The first one in particular is an in-depth interview with the designer of the World Trade Center\Twin Towers. He explains in detail the "flaws" in his design that caused the collapse...and by "flaws" it means unforeseen circumstances, IE: Planes flying into the building intentionally.
The designer has lived with guilt since then, always questioning what he could have done to prevent the collapse. Other engineers share their thoughts and comfort him with the knowledge that his designs were revolutionary for the time and that no one could have predicted what occurred on 9/11. In addition, they acknowledge that his design actually saved thousands of lives by maintaining structural stability for as long as it did.
The PBS/Nova series also debunks the 9/11 bomb theories here and goes into detail on information about the structure of metal that was used to build both towers here.
Now you know...and knowing is half the battle!