FILE - In this 1990 file photo, New York City skyline with World Trade Center's twin towers in the center. Before the towers crumbled, before the doomed people jumped and the smoke billowed and the planes hit, the collective American memory summoned one fleeting fragment of beauty: a clear blue sky. (AP Photo, File)

A test of the Tribute in Light rises above lower Manhattan, Tuesday, Sept. 6, 2011 in New York. Four World Trade Center, second from left, is under construction. The memorial, sponsored by the Municipal Art Society, will light the sky on the evening of Sept. 11, 2011 in honor of those who died ten years before in the terror attacks on the United States. (AP Photo/Mark Lennihan)

A family member of one of the victims of the crash of United Flight 93 walks along a section of Phase 1 of the permanent National Memorial following its dedication, near the crash site of Flight 93 in Shanksville, Pa., Saturday, Sept. 10, 2011. The names of the 40 victims of the crash are inscribed on the marble panels. (AP Photo/Amy Sancetta)

Pedestrians read from a wall of remembrances on a fence at ground zero in New York, Saturday, Sept. 10, 2011. The National September 11 Memorial will be dedicated on the 10th anniversary of the attacks, Sept. 11, 2011, and will be open to the public the following day.(AP Photo/Oded Balilty)

Workers scrub a September 11 Memorial waterfall pool, 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)

Former President Bill Clinton hugs former President George W. Bush after speaking during the dedication of phase 1 of the permanent Flight 93 National Memorial near the crash site of Flight 93 in Shanksville, Pa., Saturday, Sept. 10, 2011. (AP Photo/Amy Sancetta)