An American flag flies near the base of the destroyed World Trade Center in New York, in this file photo from September 11, 2001, taken after the collapse of the towers. This year's anniversary of the September 11 attacks in New York and Washington will echo the first one, with silence for the moments the planes struck and when the buildings fell, and the reading of 2,792 victims' names.

FILE PHOTO - Cars smoulder in the street as the destroyed World Trade Center burns in New York on September 11, 2001. Two hijacked commercial planes slammed into the twin towers of the World Trade Center causing both 110-story landmarks to collapse in thunderous clouds of fire and smoke.

A lone police officer sits away from where family members of the victims of the September 11 attacks on the World Trade Center were gathering at the base of the "Ground Zero," September 11, 2002.

Firemen work near the wreckage of the World Trade Center after planes crashed into each of the buildings in New York on September 11, 2001.

Courtney Ball, 19, of Sommerville, New Jersey, cries at the Flight 93 Temporary Memorial just outside Shanksville, Pennsylvania, September 11, 2005. Sunday is the fourth anniversary of the September 11, 2001 attacks on Pennsylvania, New York and Washington D.C. Pictures of the Year 2005

Pedestrians react to the World Trade Center collapse September 11, 2001. Two commercial airplanes crashed into the World Trade Center earlier. REUTERS/Stringer - PBEAHUKXYAY

A man grieves outside the World Trade Center site in New York September 11, 2006. New York is observing the fifth anniversary of the September 11 attacks on the twin towers.

A September 11, 2001 file photo shows the towers of New York's World Trade Center pour smoke shortly after being struck by hijacked commercial aircraft. New York City plans to mark the third anniversary of the attacks on the trade center with an observance at the site on September 11 with parents and grandparents of victims reading their names.

The wreck of the World Trade Center smolders in the background as a man passes a subway stop near ther World Trade Center Towers after planes crashed into each of the buildings in New York on September 11, 2001. Three hijacked planes crashed into major U.S. landmarks on Tuesday, destroying both of New York's mighty twin towers and plunging the Pentagon in Washington into flames, in an unprecedented assault on key symbols of U.S. military and financial power.

The second tower of the World Trade Center bursts into flames after being hit by a hijacked airplane in New York in this September 11, 2001 file photograph. Al Qaeda leader Osama bin Laden was killed in a firefight with U.S. forces in Pakistan on May 1, 2011, ending a nearly 10-year worldwide hunt for the mastermind of the Sept. 11 attacks. The Brooklyn bridge is seen in the foreground. REUTERS/Sara K.

FILE PHOTO - A firefighter walks amid rubble near the base of the destroyed World Trade Center in New York on September 11, 2001.

A rescue helicopter surveys damage to the Pentagon as firefighters battle flames after a hijacked airplane crashed into the U.S. military headquarters outside of Washington, in this September 11, 2001 file photo. This year's anniversary of the September 11 attacks in New York and Washington will echo the first one, with silence for the moments the planes struck and when the buildings fell, and the reading of 2,792 victims' names.

Investigators head into the debris field at the site of a commercial plane crash near Shanksville, Pennsylvania, September 11, 2001. The crash is one of four planes that were hijacked as part of a deadly and destructive terrorist plot. In the worst attack on American soil since Pearl Harbor, three hijacked planes slammed into the Pentagon and New York's landmark World Trade Center on Tuesday, demolishing the two 110-story towers that symbolize U.S.

The remaining tower of New York's World Trade Center, Tower 2, dissolves in a cloud of dust and debris about a half hour after the first twin tower collapsed September 11, 2001. Each of the towers were hit by hijacked airliners in one of numerous acts of terrorism directed at the United States September 11, 2001. The pictures were made from across the Hudson River in Jersey City, New Jersey. REUTERS/Ray Stubblebine - PBEAHUKXXBE

A picture of a person who died in the September 11th attacks at the World Trade Center are seen on a fence at the site in New York, May 3, 2011 after U.S. forces killed al Qaeda leader Osama Bin Laden in Pakistan early Monday.

Two years and two wars after the September 11 attacks, President Bush's aura of invincibility has faded and his challenge may be to keep his presidency from being more associated with Americans dying in Iraq than with his dramatic pledge at Ground Zero to battle terrorism. Bush is shown with retired firefighter Bob Beckwith (R) at the scene of the World Trade Center disaster on September 14, 2001.

A group of firefighters walk amid rubble near the base of the destroyed south tower of the World Trade Center in New York in this file photo from September 11, 2001. This year's anniversary of the September 11 attacks in New York and Washington will echo the first one, with silence for the moments the planes struck and when the buildings fell, and the reading of 2,792 victims' names.

U.S. President George W. Bush listens as White House Chief of Staff Andrew Card informs him of a second plane hitting the World Trade Center while Bush was conducting a reading seminar at the Emma E. Booker Elementary School in Sarasota, Florida, in this September 11, 2001 file photo. This year's anniversary of the September 11 attacks in New York and Washington will echo the first one, with silence for the moments the planes struck and when the buildings fell, and the reading of 2,792 victims' names.

A September 11, 2001 file photo of rescue workers carry mortally injured New York City Fire Department chaplain. A September 11, 2001 file photo shows rescue workers carrying mortally injured New York City Fire Department chaplain, the Rev. Mychal Judge, from the wreckage of the World Trade Center in New York City. The Chaplain was crushed to death by falling debris while giving a man last rites in the trade center. New York City plans to mark the third anniversary of the attacks on the trade center with an observance at the site on September 11 with parents and grandparents of victims reading their names.

The World Trade Center in New York City burns early, September 11, 2001. Both towers were hit by planes crashing into the buildings, and they collapsed a short time later.

Pedestrians and police run as a tower of the World Trade Center collapses, after two planes crashed into the complex destroying New York's mighty twin towers September 11, 2001. Loss of life is expected to be catastrophic from the collapse of the giant towers where some 50,000 people work.

People look out of the burning North tower of the World Trade Center in New York City September 11, 2001. Both towers were hit by planes crashing into the buildings. Shortly after this photo was taken this tower fell. In the worst attack on American soil since Pearl Harbor, three hijacked planes slammed into the Pentagon and New York's landmark World Trade Center on Tuesday, demolishing the two 110-story towers that symbolize U.S. financial might.