President Barack Obama and first lady Michelle Obama, joined by members of the White House staff pause during a moment of silence to mark the 11th anniversary of the Sept, 11th, Tuesday, Sept. 11, 2012, on the South Lawn of the White House in Washington. (AP Photo/Carolyn Kaster)

President Barack Obama first lady Michelle Obama, and members of the White House staff pause during a moment of silence to mark the 11th anniversary of the Sept, 11th, Tuesday, Sept. 11, 2012, on the South Lawn of the White House in Washington. (AP Photo/Carolyn Kaster)

President Barack Obama and first lady Michelle Obama join members of the White House staff during a moment of silence to mark the 11th anniversary of the Sept, 11th, Tuesday Sept. 11, 2012, on the South Lawn of the White House in Washington. (AP Photo/Carolyn Kaster)

President Barack Obama and first lady Michelle Obama walk out of the White House to join members of the White House staff before a moment of silence to mark the 11th anniversary of the Sept, 11th, Tuesday, Sept. 11, 2012, on the South Lawn in Washington. (AP Photo/Carolyn Kaster)

President Barack Obama and first lady Michelle Obama join members of the White House staff during a moment of silence to mark the 11th anniversary of the Sept, 11th, Tuesday, Sept. 11, 2012, on the South Lawn of the White House in Washington. (AP Photo/Pablo Martinez Monsivais)

WASHINGTON, DC - SEPTEMBER 11: U.S. President Barack Obama and first lady Michelle Obama observe a moment of silence with White House staff to mark the 11th anniversary of the 9/11 attacks September 11, 2012 at the South Lawn of the White House in Washington, DC. The nation is commemorating the eleventh anniversary of the September 11, 2001 attacks which resulted in the deaths of nearly 3,000 people after two hijacked planes crashed into the World Trade Center, one into the Pentagon in Arlington, Virginia and one crash landed in Shanksville, Pennsylvania. (Photo by Alex Wong/Getty Images)

President Barack Obama and first lady Michelle Obama visit Section 60, placing Challenge Coins on tombstones, Tuesday, Sept. 11, 2012, at Arlington National Cemetery in Arlington, on the 11th anniversary of the 9/11 attacks. (AP Photo/Carolyn Kaster)

President Barack Obama and first lady Michelle Obama walk through Section 60 at Arlington National Cemetery in Arlington, Va., Tuesday, Sept. 11, 2012, on the 11th anniversary of the 9/11 attacks. (AP Photo/Carolyn Kaster)

US President Barack Obama speaks during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

People sit at the Pentagon memorial after US President Barack Obama spoke at a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama greets guests after speaking at a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon during the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama pauses while speaking during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon, to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama pauses while speaking during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon, to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

A woman cries as she listens as US President Barack Obama speaks during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon during the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama listens to the National Anthem during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

People listen as US President Barack Obama speaks during a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US First Lady Michelle Obama watches as US President Barack Obama places a wreath before a memorial service at the Pentagon September 11, 2012 in Washington, DC. Obama attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon to honor the victims of the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama and US first lady Michelle Obama visit graves at Arlington National Cemetery's Section 60, September 11, 2012 in Arlington, Virginia. Obama had just attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon during the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages

US President Barack Obama and US first lady Michelle Obama visit a mass grave for those who died on October 26, 2009 in a helicopter crash while serving in Afghanistan at Arlington National Cemetery's Section 60 September 11, 2012 in Arlington, Virginia. Obama had just attended the memorial service near where American Airlines flight 77 was crashed into the Pentagon during the September 11th terrorist attacks on the United States. AFP PHOTO/Brendan SMIALOWSKIBRENDAN SMIALOWSKI/AFP/GettyImages