Flames, debris and smoke burst from the south tower of the World Trade Center as it is struck by United Airlines Flight 175 on Sept. 11, 2001. Although the south tower was the second to be struck, it would be the first to collapse.
The South Tower of the World Trade Center explodes in flames after being hit by the hijacked airliner now universally known as "the second plane," United Airlines Flight 175, September 11, 2001. Ground Zero
Rick Rescorla: The Hero Who Saved 2,700 Lives on 9/11 - he began the orderly evacuation of Morgan Stanley's 2,700 employees on twenty floors of World Trade Center Tower 2, and 1,000 employees in WTC 5.
Moments after United Airlines Flight 175, with 56 passengers (including the 5 hijackers) and 9 crew members, struck the South Tower of the World Trade Center between floors 77 and 85 on September 11, 2001, in New York City. (Spencer Platt/Getty Images)