United Airlines Flight 175 approaches the South Tower of World Trade Center in New York City on Sept. 11, 2001, shortly after the North Tower is hit.
An explosion is seen at the World Trade Center's South Tower after United Airlines Flight 175 crashed in New York City.
Bystanders react to explosion at the World Trade Center towers after 9/11 terrorist attack.
Smoke billows from the World Trade Center Towers after both downtown buildings are hit by jumbo jets.
At 9:59 a.m. on Sept. 11, 2011, the South Tower collapses.
A man and woman flee through dust and debris from the collapsed World Trade Center towers.
Smoke and dust from collapse of the World Trade Center rises behind Statue of Liberty.
Pedestrians walk through the dust and debris after 9/11 World Trade Center attacks in New York City. (Credit: GULNARA SAMOILOVA/AP)
Rescue workers search rubble of collapsed World Trade Center towers after terrorist attacks.
Women hold U.S. flags and candles at a memorial service, one of many held across the nation, for victims of the terrorist attacks.
A New York City Firefighter waves the American flag during a protest against planned scaleback of recovery efforts at the site of collapsed World Trade Center towers.
A Port Authority police officer pays his respects during the Ground Zero three-year anniversary ceremony.
The famous Cross formed by beams from the original World Trade Center stands at the World Trade Center site in New York.
Members of the New York Fire Department open an American flag prior to the Ground Zero five-year anniversary ceremony.
George W. Bush, as U.S. President, first lady Laura Bush, and their military escort lay a memorial wreath in a reflecting pool at Ground Zero.
An aerial view of the World Trade Center site in Lower Manhattan years after the 9/11 attack.
The "Tribute in Light" shines over the Manhattan skyline six years after 9/11.
A young boy kneels at the edge of a reflecting pool during a memorial service commemorating the ninth anniversary of the September 11th terrorist attacks on the World Trade Center.
Work continues at the World Trade Center and Freedom Tower site.
President Barack Obama pauses at the National Sept. 11 Memorial at Ground Zero, New York City, in May 2011.