A fourth jetliner crash-landed on an empty field in Pennsylvania.
By the time the carnage was over, terrorists had killed 2,977 people in one of the deadliest attacks on American soil.
People, buildings and planes fell from the sky. Terrified strangers became friends as Americans united on a day that changed the world forever.
Today, like every other anniversary after it happened , a resilient nation will look forward to the future, but remember the past.
Here are the main events that will pay tribute to the many lives lost on September 11, 2001:
GROUND ZERO
At the World Trade Center, where most of the victims were, bagpipers and drummers will provide solemn tunes to accompany the ceremony.
The names of the dead will echo as those gathered read them out and mark moments of silence at the specific times when the planes struck and the towers tumbled.
The first plane hit the north tower at 8:46 a.m. The second one struck at 9:03 a.m.
In this attack, 2,753 people died when terrorists intentionally crashed American Airlines Flight 11 and United Airlines Flight 175 in the north and south towers.
Of those who perished in these attacks and the collapse of the towers, more than 300 were firefighters who ran into the building while others rushed out.
Dozens more were police officers.
PENNSYLVANIA
Hundreds of miles away, a passenger jetliner crashed near Shanksville at 10:03 a.m.
Crowds will mark a moment of silence at that exact time and read out the names of the victims.
Forty passengers and crew members aboard United Airlines Flight 93 died when the plane crashed into the field. It is believed that the hijackers crashed the plane in that location, rather than their unknown target after the passengers and crew attempted to retake control of the flight deck.
PENTAGON
At sunrise a flag will be unfurled over the side of the Pentagon.
Shortly after, a private memorial ceremony will include wreath laying and remarks by Secretary of Defense Ash Carter. It will be attended by relatives of the victims.
At the White House, President Barack Obama will observe a moment of silence on the south lawn to mark the anniversary of the attacks. He is also scheduled to visit Fort Meade in Maryland, where he will talk with troops and express his appreciation.
"The President very much values face time with troops -- listening, asking, and answering questions, and he looks forward to taking time on the anniversary of 9/11 to engage directly with service members," Pentagon spokesman Peter Cook said.
At the Pentagon, 184 people were killed when hijacked American Airlines Flight 77 crashed into the building. It struck at 9:37 a.m.
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