The morning of Sept. 11, 2001, began as a bright, sunny early fall day. That morning, former President George Bush was reading to a group of students at Emma E. Booker Elementary School in Sarasota, Fla., when he was told that a plane had hit the World Trade Center in New York. Initially, an accident seemed the most likely explanation. But the reading event was cut abruptly short after an aide whispered to Bush that a second plane had hit the other Twin Tower.
The South Tower of the World Trade Center bursts into flames after being struck by hijacked United Airlines Flight 175 at 9:03 a.m. on Sept. 11, 2001. The North Tower continues to burn following an earlier attack by another hijacked airliner, American Airlines Flight 11.
This aerial shot by the New York City Police Department's Aviation Unit shows one of the towers burning from above.
People look out of the burning North Tower of the World Trade Center. Shortly after this photo was taken, the tower collapsed.
At 9:37 a.m., five hijackers flew American Airlines Flight 77 into the Pentagon. Here, a rescue helicopter surveys damage as firefighters try to battle the intense flames.
A fourth hijacked flight, United Airlines Flight 93, crashed near Shanksville, Pa., at 10:03 a.m. after passengers reportedly fought with the hijackers. Here, investigators comb what little debris was left for the flight data recorder.
At 9:59 a.m., the South Tower of the World Trade Center collapsed, leaving a billowing cloud of toxic smoke and debris in its wake as seen in this photo taken by the U.S. Coast Guard.
The North Tower collapsed at 10:28 a.m. after burning for 102 minutes
More smoke and debris flood the streets of Manhattan shortly after the collapse of the North Tower.
This shot by the NYPD's Aviation Unit captures the collapse of the North Tower as it falls.
A man passes by a subway stop in this picture that captures the surreal atmosphere of Manhattan shortly after both towers collapsed.
In one of the more iconic 9/11 images, rescue workers carry the body of New York Fire Department Chaplain Mychal Judge, on a stretcher in the middle of this photo, after he was killed in the collapse of the South Tower.
Here, one of many workers covered in dust and debris walks the streets near the World Trade Center following its collapse.
New York City firefighters pour water on the wreckage of 7 World Trade Center, which collapsed later that afternoon after burning most of the day.
A crane lifts rubble from the ruins of the World Trade Center.
Firemen carry an injured man from the World Trade Center after both towers collapsed.
Workers walk near the wreckage of the World Trade Center.
Iron Workers Danny Parker and Eddie Scandel, on top of the rubble at Ground Zero, cut a piece of steel that once was the 74th floor of the World Trade Center.
In one of the most iconic images to emerge in the days after 9/11, former President George Bush stands on top of rubble at the World Trade Center site with retired New York City fireman Bob Beckwith. "I can hear you," he told rescue workers. "The rest of the world hears you. And the people who knocked these buildings down will hear all of us soon."
A man places his hand on a small part of a huge wall showing photos of missing people outside Bellevue Hospital in New York City -- a solemn reminder of the number of lives lost that day.