PHOTOS: 9/11 attacks on the World Trade Center in New York City

NEW YORK -- New York City will never forget September 11, 2001.

Nearly 3,000 people were killed the day when hijackers used two passenger planes as weapons to topple the twin towers of the World Trade Center and another to attack the Pentagon. A fourth hijacked plane went down in flames when heroic passengers fought back to bring it down in rural Pennsylvania before it could strike another high-profile target, probably in Washington.

Looking at images from that day may be difficult for some. Smoke, flames, and debris engulf the twin towers, as people on the ground, some covered in ash, watch in horror. Familiar landmarks, like subway stations, are completely destroyed, and one image shows the iconic city skyline covered in smoke.

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