9/11 attacks: The defining images from that day of horror

The rubble of the World Trade Center smoulders following a terrorist attack Sept. 11, 2001 in New York. A hijacked plane crashed into and destroyed the landmark structure. (Porter Gifford/Corbis via Getty Images)

Hijacked United Airlines Flight 175 from Boston crashes into the south tower of the World Trade Center and explodes at 9:03 a.m. on Sept. 11, 2001 in New York City. The crash of two airliners hijacked by terrorists loyal to al Qaeda leader Osama bin Laden and subsequent collapse of the twin towers killed some 2,800 people. (Spencer Platt/Getty Images)

Rescue workers sift through the wreckage of the World Trade Center Sept. 13, 2001 in New York City, two days after two hijacked airplanes slammed into the twin towers, levelling them in an alleged terrorist attack. (Mario Tama/Getty Images)

A military helicopter flies in front of the Pentagon Sept.m14, 2001 in Arlington, Virginia at the impact site where a hijacked airliner crashed into the building. (Stephen J. Boitano/Getty Images)

Policemen and firemen run away from the huge dust cloud caused as the World Trade Center's Tower One collapses after terrorists crashed two hijacked planes into the twin towers, Sept. 11, 2001 in New York City. (Stephen J. Boitano/Getty Images)

New York firefighters amid the rubble of the World Trade Center following the 9/11 attacks. (Universal History Archive/Universal Images Group via Getty Images)

Aerial view of the damage to the building following an attack at the Pentagon on Sept. 11, 2001 in Arlington, Va., in this undated image. American Airlines Flight 77 was hijacked by al Qaeda terrorists who flew it in to the building killing 184 people. (Federal Bureau of Investigation via Getty Images)

Aerial view after the 9/11 attacks on the World Trade Center Site in New York.  (Universal History Archive/Universal Images Group via Getty Images)

A firefighter breaks down after the World Trade Center buildings collapsed Sept. 11, 2001 after two hijacked airplanes slammed into the twin towers in a terrorist attack. (Mario Tama/Getty Images)

A New York firefighter amid the rubble of the World Trade Center following the 9/11 attacks.  (Universal History Archive/Universal Images Group via Getty Images)

A New York rescuer worker amid the rubble of the World Trade Center following the 9/11 attacks.  (Universal History Archive/Universal Images Group via Getty Images)

A fiery blasts rocks the World Trade Center after being hit by two planes Sept. 11, 2001 in New York City. (Spencer Platt/Getty Images)

A New York firefighter amid the rubble of the World Trade Center following the 9/11 attacks.  (Universal History Archive/Universal Images Group via Getty Images)

The Manhattan skyline following the terrorist attacks on the World Trade Center.  (Universal History Archive/Universal Images Group via Getty Images)

A New York firefighter amid the rubble of the World Trade Center following the 9/11 attacks.  (Universal History Archive/Universal Images Group via Getty Images)

Smoke billows from the fire following an attack at the Pentagon on Sept. 11, 2001 in Arlington, Va., in this undated image. American Airlines Flight 77 was hijacked by al Qaeda terrorists who flew it in to the building killing 184 people. (Federal Bureau of Investigation via Getty Images)

Smoke pours from the twin towers of the World Trade Center after they were hit by two hijacked airliners in a terrorist attack Sept. 11, 2001 in New York City. (Robert Giroux/Getty Images)