Defining images from the 9/11 attacks

The second tower of the World Trade Center bursts into flames after being hit by a hijacked airplane in New York September 11, 2001. REUTERS/Sara K. Schwittek

Pedestrians react to the World Trade Center collapse September 11, 2001. REUTERS/Stringer

Then-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,...more

People look out of the burning North Tower of the World Trade Center in New York City September 11, 2001. REUTERS/Jeff Christensen

A helicopter flies past the World Trade Center after a commercial plane slammed into it in New York City, September 11, 2001. REUTERS/Jeff Christensen

The towers of the World Trade Center pour smoke shortly after being struck by hijacked commercial aircraft September 11, 2001. REUTERS/Brad Rickerby

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, September 11, 2001. REUTERS/Larry Downing

The remaining tower of 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. REUTERS/Ray Stubblebine

Pedestrians and police run as a tower of the World Trade Center collapses, September 11, 2001. REUTERS/Reuters TV

A group of firefighters walk amid rubble near the base of the destroyed South Tower of the World Trade Center in New York, September 11, 2001. REUTERS/Peter Morgan

Rescue workers carry mortally injured New York City Fire Department chaplain, the Rev. Mychal Judge, from the wreckage of the World Trade Center in New York City September 11, 2001. The chaplain was crushed to death by falling debris while giving a...more

Cars smolder in the street as the destroyed World Trade Center burns in New York, September 11, 2001. REUTERS/Peter Morgan

A man walks in the street near the World Trade Center towers in New York City, September 11, 2001. REUTERS/Shannon Stapleton

Investigators head into the debris field at the site of a commercial plane crash near Shanksville, Pennsylvania, September 11, 2001. REUTERS/Jason Cohn

The wreck of the World Trade Center smolders in the background as a man passes a subway stop near the World Trade Center towers, September 11, 2001. REUTERS/Peter Morgan

An American flag flies near the base of the destroyed World Trade Center, September 11, 2001. REUTERS/Peter Morgan

President George Bush (2nd R) is pictured with Vice President Dick Cheney (R) and senior staff in the President's Emergency Operations Center in Washington in the hours following the attacks, September 11, 2001. REUTERS/U.S. National Archives/Handout...more

Firemen work near the wreckage of the World Trade Center, September 11, 2001. REUTERS/Peter Morgan

Workers at St. Vincent's Hospital wait for injured people from the World Trade Center towers in New York, September 11, 2001. REUTERS/Peter Morgan

A firefighter walks amid rubble near the base of the destroyed World Trade Center in New York, September 11, 2001. REUTERS/Peter Morgan

The damaged area of the Pentagon building, where a commercial jetliner slammed into it September 11, is seen in the early morning at sunrise with the U.S. Capitol Building in the background, September 16, 2001. REUTERS/Larry Downing

A New York City fireman calls for more rescue workers to make their way into the rubble of the World Trade Center, September 15, 2001. REUTERS/U.S. Navy/Journalist 1st Class Preston Keres

Then-President George W. Bush is shown with retired firefighter Bob Beckwith (R) at the scene of the World Trade Center disaster, September 14, 2001. REUTERS/Win McNamee

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. REUTERS/Mike Segar

A man grieves outside the World Trade Center site in New York, September 11, 2006. REUTERS/Shannon Stapleton

A picture of a person who died in the September 11 attacks at the World Trade Center is 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. REUTERS/Shannon Stapleton

Courtney Ball, 19, of Sommerville, New Jersey, cries at the Flight 93 Temporary Memorial just outside Shanksville, Pennsylvania, September 11, 2005. REUTERS/Jason Cohn