11.   A little more than hour later, the south tower collapsed, and the north tower folowed it.

12.   A recently returned humanitarian aid worker, I had rushed downtown when the towers collapsed.

13.   After the towers collapsed, every phone went dead at police headquarters downtown, the home of its own command center.

14.   After the towers collapsed, one mourning friend vowed a bloody vengeance.

15.   Both towers collapsed.

16.   But eventually, the fires softened the steel structures, and the twin towers collapsed in a terrifying avalanche.

17.   But then the second tower collapsed.

18.   By the time the south tower collapsed, he was on the Brooklyn Bridge along with thousands of others fleeing Lower Manhattan.

19.   Byline TK ATTACK-COLLAPSE A look at why and how WTC towers collapsed.

20.   By the time New York City Fire Department Marshal Kevin James arrived on the scene as backup from his Brooklyn base, the twin towers had collapsed.