On that fateful day, jet planes took down the World Trade Center in New York City and struck the Pentagon in Washington D.C., while another hijacked plane went down in a field in Pennsylvania.
It was the largest terrorist plot to ever unfold on American soil.
One aspect of that day that has failed to die down is the deluge of conspiracy theories that continue to spread about 9/11.
Were the planes swapped out with remote aircraft filled with explosives? Was the plane that went down in Pennsylvania struck by a missile?
AllTime10s presents 10 disturbing 9/11 conspiracy theories below.