According to the 9/11 Memorial website, it was estimated that there were between 16,400 and 18,000 people at the World Trade Center (WTC) complex when the planes struck the buildings.

PHOTOS: 9/11 Tribute in Light through the years . ... September 11 still shapes American policy, politics and everyday experiences in places from airports to office buildings, ...

It's been 21 years since the the 9/11 terrorist attacks that shook the world. Here, we look at the most powerful photos from September 11 and the days after.

On Sept. 11, 2001, four U.S. airliners were highjacked in a coordinated attack by the terrorist organization al-Qaeda against the United States, crashing into the Twin Towers of the World Trade Cen… ...

This September 11 attack timeline shows how the events on 9/11 unfolded ... People run from the collapse of one of the twin towers of New York's World Trade Center in this Sept. 11, 2001, file photo.

These vivid images offer a special glimpse into the events in and around the Pentagon on September 11, 2001. ... Related: Check Out These 20 Rarely Seen 9/11 Photos.

2 A trader photographs a list of names displayed on a screen of victims of the September 11, 2001 attacks on the 22nd anniversary, on the floor at the New York Stock Exchange, in New York City.

New York City remembers those lost but not forgotten on the 23rd anniversary of the September 11 terrorist attacks on the World Trade Center. Family members and loved ones of the victims attended ...

On Wednesday morning, September 11, remembrances were held in Greenville, Spartanburg, and Clemson. Nearly 50 people gathered at Boiling Springs Fire District Station Headquarters in Greenville to ...

It's easy as the years pass to treat September 11 like just another day. To get caught up in the hurry and activity of life and work and family, and forget to pause to remember. But it's important ...