On Sept. 11, 2001, four planes crashed: two in New York City, one outside Washington D.C., and one in a remote Pennsylvania field.
That last plane, United Airlines Flight 93, was diverted from its likely target by its passengers and crew. All 40 were killed.
Now their loved ones have dedicated the final piece of a memorial commemorating them: the Tower of Voices.
The newest and last feature of the Flight 93 National Memorial is 93 feet tall, and designed to hold 40 massive wind chimes.