Why did the Twin Towers collapse straight down?

Related questions

1. How many firemen died in 911?
2. Is the World Trade Center and the twin towers the same?
3. Why is it called Ground Zero?
4. How long did it take for the twin towers to collapse?
5. What is considered ground zero?
6. Did they find any survivors ground zero?
7. What does the phrase ground zero mean?
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Tags

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2. #fires
3. #known
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5. #reasons
6. #primarily
7. #building
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13. #water
14. #remarkable
15. #center
16. #trade
17. #collapse
18. #collapsing
19. #because
20. #world
21. #instance