** FILE ** The National September 11 Memorial and Museum is under construction, lower right, Tuesday, May 3, 2011, at the World Trade Center site in New York. The memorial is scheduled to open to the public in time for the 10th anniversary to be held Sept. 11, 2011. (AP Photo/Mark Lennihan)
President Barack Obama, center, shakes hands with first responders as he arrives for a wreath laying ceremony at the September 11 Memorial at the World Trade Center site in New York, Thursday, May 5, 2011. (AP Photo/Mark Lennihan)
President Barack Obama, accompanied by a New York City Police officer, New York City Firefighter, and Port Authority officers, carries a wreath to be placed at the World Trade Center site, in New York, Thursday, May 5, 2011. (AP Photo/Richard Drew)
On Sept. 11, 2001, the twin towers of the World Trade Center burn and later collapse after hijacked planes deliberately crashed into them in New York City. A multimillion-dollar reward was offered for now-dead terrorist Osama bin Laden after the attacks.
Edward Vinciguerra (left), a structural supervisor with the Port Authority of New York & New Jersey, watches a 5½-foot-long fragment of an I-beam, salvaged from the World Trade Center, being loaded on a truck on Wednesday. (Associated Press)
Archeologists, center, take measurements of the wood hull of a 32-foot-long (9.75 meters) 18th century boat at the World Trade Center site, Thursday, July 15, 2010 in New York. (AP Photo/Mark Lennihan)