On 11 September 2001 (known as 9/11 in America), Islamist extremists hijacked four planes that were flying above the US.
Two of them were flown into the twin towers of the World Trade Center in New York.
Another was crashed into the Pentagon, the top military building in the capital city, Washington DC.
The fourth plane crashed into a field, 80 miles from Pittsburgh, Pennsylvania.
The attacks shocked the world. It was the biggest terrorist attack ever on America.
The twin towers of the World Trade Center were the highest buildings in New York, and famous throughout the world.
Their destruction transformed the city's skyline and would have a huge impact on the rest of the world for years to come.
8.38am US time (1.38pm UK time)
US air traffic controllers confirmed a passenger plane, American Airlines Flight 11, had been hijacked.
8.45am US time (1.45pm UK time)
Flight 11 crashed into the north tower of the World Trade Center in New York.
9.03am US time (2.03pm UK time)
A second hijacked passenger plane, United Airlines Flight 175, crashed into the south tower of the World Trade Center.
9.30am US time (2.30pm UK time)
US President George Bush made a TV statement to people in the US, saying: "We have had a national tragedy.
Two aeroplanes have crashed into the World Trade Center in an apparent terrorist attack on our country."
9.37am US time (2.37pm UK time)
A third hijacked plane, American Airlines Flight 77, crashed into the US military headquarters in Washington DC.
9.59am US time (2.59pm UK time)
The south tower of the World Trade Center collapsed to the ground in New York.
Many people were killed, including emergency workers and firefighters who went into the building to help people.
10.03am US time (3.03pm UK time)
A fourth hijacked plane, United Airlines Flight 93, crashed into a field.
10.28am US time (3.28pm UK time)
The north tower of the World Trade Center collapsed to the ground as well. Many more people lost their lives.