SHANKSVILLE, Pa. (WTAJ) — September 11th marks 21 years since the terrorist attacks that sent shock waves around the world and changed the United States of America forever.

In remembrance of this day, a ceremony is being held at the Flight 93 National Memorial in Shanksville to honor those who died and sacrificed their lives as they fought hijackers, forcing the plane to crash into an open field.

The hour-long service began at 9:45 a.m. and a replay can be watched in the video player above.

Starting at 10:03 a.m., when Flight 93 crashed, the names of each passenger and crew member will be read and the Bells of Remembrance will ring in their memory. First Lady Dr. Jill Biden is also scheduled to speak at the ceremony.

On Tuesday, Sept. 11, 2001, United Airlines Flight 93 was flying from New Jersey to San Francisco when Al-Qaeda terrorists hijacked the aircraft and began to head for Washington, D.C. The hijackers intended target was believed to be the U.S. Capitol building.

The 40 passengers on the plane decided to take action after hearing about other hijacked planes that crashed into the World Trade Center in New York City and the Pentagon near Washington, D.C. After calling their loved ones to say their final goodbyes, the passengers fought back and tried to regain control of the plane.

The passenger’s assault forced the terrorists to crash the plane short of its intended destination into an open field near Shanksville, Pennsylvania. Everyone aboard Flight 93 died in the crash.

21 years later, the passengers and crew are remembered for their sacrifice and heroic actions that saved countless other lives.

More information about the Flight 93 National Memorial and the remembrance ceremony can be found on the National Park Service website.