On the morning of Sept. 11, 2001, terrorists hijacked four passenger planes and crashed them into the World Trade Center in New York, the Pentagon in Washington, D.C., and a field in Somerset County, Pennsylvania.
Nearly 3,000 lives were lost.
To mark the 15th anniversary of one of the most devastating days in U.S. history, ABC News revisited the sites of the plane crashes and captured 360-degree videos of the powerful memorials that have since been erected to honor the thousands who died that day.
If you are on a desktop or laptop, you can click and drag your mouse or use your keyboard's arrow keys to tour the memorials. For smartphones or tablets, make sure to download the Facebook app to view the videos. You can then move your phone or fingers to tour the memorials.
The National 9/11 Memorial at World Trade Center in New York
At the World Trade Center (WTC) in Lower Manhattan, 2,753 were killed after American Airlines Flight 11 and United Airlines Flight 175 were hijacked and intentionally crashed into the North and South Towers.
The victims ranged in age from 2 to 85 years old.
At the memorial, two reflecting pools sit where the Twin Towers once stood. The names of every person who died during the attacks are inscribed into bronze panels edging the memorial pools.
The National 9/11 Pentagon Memorial
At the Pentagon in Washington, D.C, 184 people were killed when hijacked American Airlines Flight 77 crashed into the western facade of the building.
The memorial features cantilevered benches where the 184 victims' names and age are permanently inscribed. Below the benches are lighted pools of water.
There are also 85 crape myrtle trees around the memorial units, which designers planted to provide a canopy of shade in years to come.
Flight 93 National Memorial in Somerset County, Pennsylvania
All 40 passengers and crew members aboard the hijacked United Airlines Flight 93 were killed after it was crashed into an open field in Stonycreek Township, Somerset County, Pennsylvania.
The hijacked plane was on course to hit the nation's capitol, but because of the quick and determined actions of the passengers and crew, Flight 93 was the only one of the four hijacked aircraft that failed to reach the terrorists' intended target on 9/11.
The field memorial features 40 engraved marble panels that make up the Wall of Names, honoring the 40 lives lost. There is also a stark black wall that marks the debris field and a sandstone boulder that marks the impact zone.
Editor’s note: An earlier version of this story incorrectly stated that three planes were hijacked as part of the 9/11 terrorist attacks. The story has been amended to note that four planes were hijacked on that day.