Roses lay on the north pool placed by Dawn Nelson in memory of her sister Ann N. Nelson and Aaron Horwitz at the National September 11 Memorial during a ceremony marking the 10th anniversary of the attacks at World Trade Center, Sunday, Sept. 11, 2011 in New York. (AP Photo/Mary Altaffer)

President Barack Obama and first lady Michelle Obama former President George W. Bush and former first lady Laura Bush visit North Memorial Pond at the National Sept. 11th Memorial Sunday, Sept., 11, 2011, in New York. (AP Photo/Pablo Martinez Monsivais)

One World Trade Center overlooks the ground zero memorial site, Saturday, Sept. 10, 2011, in New York. Ceremonies will be held at the site Sunday for the 10th anniversary of the September 11 attacks. (AP Photo/Mark Lennihan)

The World Trade Center is aglow in light, Sunday, Sept. 11, 2011 in New York. One World Trade Center is on the left. A decade has passed since the Sept. 11 attacks when terrorists crashed hijacked planes into the World Trade Center and the Pentagon, and a fourth plane crashed into a field in rural western Pennsylvania.(AP Photo/Mark Lennihan)

President Barack Obama walks past a reflecting pool with his wife Michelle, and former President George W. Bush and his wife, Laura, as they arrive for the ceremony marking the 10th anniversary of the attacks, Sunday, Sept. 11, 2011 in New York. Two reflecting pools built over the towers' footprints, part of a Sept. 11 memorial that was to open later in the day for relatives of the victims. (AP Photo/Mark Lennihan)

Christine Corday puts final touches on the name panels on the north pool at the National September 11 Memorial before a ceremony marking the 10th anniversary of the attacks at the World Trade Center site Sunday, Sept. 11, 2011 in New York. (AP Photo/Mary Altaffer)

Maria Crifasi pays respects at the site where her sister Lucia Crifasi's name is engraved at the north pool at the National September 11 Memorial for a ceremony marking the 10th anniversary of the attacks at World Trade Center, Sunday, Sept. 11, 2011 in New York. (AP Photo/Mary Altaffer)

President Barack Obama and first lady Michelle Obama, former President George W. Bush and former first lady Laura Bush bow their heads during a moment of silence at the Sept. 11 10th Anniversary Commemoration Ceremony at Ground Zero in New York, Sunday, Sept., 11, 2011 in New York. (AP Photo/Pablo Martinez Monsivais)

President Barack Obama and first lady MIchelle Obama greet family members of those who died during the attacks on the World Trade Center at the National September 11 Memorial during a ceremony marking the 10th anniversary of the attacks at World Trade Center, Sunday, Sept. 11, 2011 in New York. (AP Photo/Mary Altaffer)

Robert Peraza, who lost his son Robert David Peraza in the attacks at the World Trade Center, pauses at his son's name at the North Pool of the 9/11 Memorial before the 10th anniversary ceremony at the site, Sunday Sept. 11, 2011, in New York. (AP Photo/Justin Lane, Pool)