Tower of Voices

bioLINIA and Paul Murdoch Architects / AP

The 16th anniversary of United Flight 93's crash into a Pennsylvania field during the 9/11 terrorist attacks will mark the beginning of the end of a $46 million effort to transform the rural Pennsylvania crash site into a national memorial park.

Ground was broken Sunday on the final element of the Flight 93 National Memorial — a 93-foot tall Tower of Voices.