Photo by Jeff Swensen / Getty Images

Flight 93 Memorial 2011 dedication

Former US President George W. Bush speaks during the dedication of the Flight 93 memorial in Shanksville, Pennsylvania, on September 10, 2011. A long white stone wall bearing the names of those who struggled with Al-Qaeda terrorists on the fourth airliner to be hijacked on September 11, 2001 was unveiled on the rural Pennsylvania field where the Boeing 757 crashed. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

SHANKSVILLE, UNITED STATES: World War II veteran Charles Vizzini (R) stands near a plaque commemorating the crew and passengers of United Airlines flight 93 during a dedication ceremony at a site overlooking the field where United Airlines flight 93 crashed in Shanksville, Pennsylvania on 11 September, 2001. Today marks the six-month anniversary of the September 11 terrorist attacks. AFP Photo/David Maxwell (Photo credit should read DAVID MAXWELL/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

A US flag is seen as some 3,000 candles are lit at the newly dedicated Flight 93 memorial at the site where the United plane crashed in Shanksville, Pensylvania, on September 10, 2011 as the US prepares to mark the tenth anniversary of the September 11, 2001 attacks. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

People climb a hill as clouds appear in the background after a brief rainstorm following a ceremony marking the tenth anniversary of the September 11, 2001 attacks at the newly dedicated Flight 93 memorial in Shanksville, Pennsylvania, on September 11, 2011 . AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

A family member of a victim of United flight 93 attends the dedication of the Flight 93 memorial in Shanksville, Pennsylvania, on September 10, 2011. A long white stone wall bearing the names of those who struggled with Al-Qaeda terrorists on the fourth airliner to be hijacked on September 11, 2001 was unveiled on the rural Pennsylvania field where the Boeing 757 crashed. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

SHANKSVILLE, PA - SEPTEMBER 10: Ken Salazar, George and Laura Bush, Bill Clinton and Vice President Joe Biden bow their heads in prayer at the Flight 93 Memorial Saturday September 10, 2011 in Shanksville, Pa. The Flight 93 National Memorial was formally dedicated with Presidents Goerge W. Bush and Bill Clinton, Vice President Joe Biden, Secretary of the Interior Ken Salazar, Speaker of the House John Boehner and members of Congress in attendance. A estimated crowd of 5000 watched as the memorial wall was unveiled with the names of the 40 passengers on the plane that crashed on September 11, 2001. (Photo by Jeff Swensen/Getty Images)

Flight 93 Memorial 2011 dedication

A woman displays a US flag with an inscription reading " Our nation will eternally honor the heroes of Flight 93" as people listen to speakers during a ceremony marking the tenth anniversary of the September 11, 2001 attacks at the newly dedicated Flight 93 memorial where the hijacked airliner fell in Shanksville, Pensylvania, on September 11, 2011. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

SHANKSVILLE, PA - SEPTEMBER 10: Members of the Shanksville Volunteer Fire Department listen to the proceedings at the Flight 93 Memorial Saturday September 10, 2011 in Shanksville, Pa. The Flight 93 National Memorial was formally dedicated with Presidents Goerge W. Bush and Bill Clinton, Vice President Joe Biden, Secretary of the Interior Ken Salazar, Speaker of the House John Boehner and members of Congress in attendance. A estimated crowd of 5000 watched as the memorial wall was unveiled with the names of the 40 passengers on the plane that crashed on September 11, 2001. (Photo by Jeff Swensen/Getty Images)

Flight 93 Memorial 2011 dedication

Flowers are placed on the section of the Wall of Names dedicated to Captain Jason Dahl at the newly dedicated Flight 93 memorial at the site where the United plane crashed, marked by the boulder in te background, in Shanksville, Pensylvania, on September 10, 2011 as the US prepares to mark the tenth anniversary of the September 11, 2001 attacks. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

SHANKSVILLE, PA - SEPTEMBER 10:Jonathon Daniels, 9, son of Shanksville Fireman, Sean Daniels, takes a photo of the proceedings at the Flight 93 Memorial for the dedication ceremony Saturday, September 10, 2011. The Flight 93 National Memorial was formally dedicated with Presidents Goerge W. Bush and Bill Clinton, Vice President Joe Biden, Secretary of the Interior Ken Salazar, Speaker of the House John Boehner and members of Congress in attendance. A estimated crowd of 5000 watched as the memorial wall was unveiled with the names of the 40 passengers on the plane that crashed on September 11, 2001. (Photo by Jeff Swensen/Getty Images)

Flight 93 Memorial 2011 dedication

SHANKSVILLE, PA - SEPTEMBER 10: Ken Salazar, George and Laura Bush, Bill Clinton and Vice President Joe Biden walk along the Wall of Names at the Flight 93 Memorial Saturday September 10, 2011 in Shanksville, Pa. The Flight 93 National Memorial was formally dedicated with Presidents Goerge W. Bush and Bill Clinton, Vice President Joe Biden, Secretary of the Interior Ken Salazar, Speaker of the House John Boehner and members of Congress in attendance. A estimated crowd of 5000 watched as the memorial wall was unveiled with the names of the 40 passengers on the plane that crashed on September 11, 2001. (Photo by Jeff Swensen/Getty Images)

Flight 93 Memorial 2011 dedication

Singer Sarah McLachlan performs during the dedication of the Flight 93 memorial in Shanksville, Pennsylvania, on September 10, 2011. A long white stone wall bearing the names of those who struggled with Al-Qaeda terrorists on the fourth airliner to be hijacked on September 11, 2001 was unveiled on the rural Pennsylvania field where the Boeing 757 crashed. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

People sing the national anthem at the start of the dedication of the Flight 93 memorial in Shanksville, Pennsylvania, on September 10, 2011. Flight 93 crashed near Shanksville on September 11, 2001. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

A woman attends the dedication of the Flight 93 memorial in Shanksville, Pennsylvania, on September 10, 2011. A long white stone wall bearing the names of those who struggled with Al-Qaeda terrorists on the fourth airliner to be hijacked on September 11, 2001 was unveiled on the rural Pennsylvania field where the Boeing 757 crashed. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)

Flight 93 Memorial 2011 dedication

US President Barack Obama pays his respects with his wife Michelle after laying a wreath at the Wall of Names during a ceremony marking the tenth anniversary of the September 11, 2001 attacks at the newly dedicated Flight 93 memorial at the site where the United plane crashed, in Shanksville, Pensylvania, on September 11, 2011. AFP PHOTO/Nicholas KAMM (Photo credit should read NICHOLAS KAMM/AFP/Getty Images)