United Airlines Flight 175 closes in on World Trade Center Tower 2 just before impact.
Smoke pours off World Trade Center Tower 1 and flames explode from Tower 2 as it is struck by American Airlines Flight 175.
A fiery blast rocks the south tower of the World Trade Center as the hijacked United Airlines Flight 175 from Boston crashes into the building.
The south tower of the World Trade Center collapses.
A firefighter breaks down after the World Trade Center buildings collapsed.
An American flag flies in the foreground as one of the World Trade Center towers burns in the background.