11.   A torrent of paper blew through the windows of a second-floor apartment on Liberty Street across from the south tower.

12.   About the eighth floor, they heard the heavy rumble of the south tower collapsing and were told to get out.

13.   Across the way in the south tower, Paul Boucher, a designer at the Washington Group, walked to a north-facing window to investigate the commotion.

14.   After all, the south tower where she worked had collapsed.

15.   But the south tower, roofless, has continued to pose a problem and in recent years it has been swathed in protective netting.

16.   By the time they all got there, the second plane was approaching the south tower, and cell phones had gone dead.

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

18.   Colleagues said he disappeared in the rubble when the south tower collapsed, just moments after he tried to help a woman who had fainted.

19.   During the next few months, the cleanup will focus on a heap that used to be the part of the south tower.

20.   Eighteen minutes later, the south tower was struck.