Thirteen years ago, during the terrorist attacks of September 11, 2001, a passenger revolt against the hijackers of United Airlines Flight 93 resulted in the aircraft crashing into a field just outside Shanksville, Pennsylvania, killing all 44 people aboard (including the four hijackers). Authorities believe this fourth hijacked aircraft was also to be used as a missile, targeting either the U.S. Capitol Building or the White House. In the years since, a national memorial at the Pennsylvania crash site has been under planning and construction, now due to be completed sometime in late 2015. Phase one was completed in 2011, including the "wall of names," 40 granite slabs standing eight feet tall, inscribed with the names of the passengers and crew of Flight 93. On this 13th anniversary of that tragic day, here are a few recent images of the Flight 93 National Memorial.