Two of the hijacked planes were deliberately flown into the Twin Towers of the World Trade Center in New York City.
Twenty-five years ago today, 19 terrorists associated with al Qaeda hijacked four commercial aircraft and carried out coordinated attacks that killed 2,977 people in the United States.
Two of the hijacked planes were deliberately flown into the Twin Towers of the World Trade Center in New York City.
A third was crashed into the Pentagon, the headquarters of the US Department of Defense, in Arlington, Virginia.
The fourth plane, United Airlines Flight 93, crashed into an open field near Shanksville, Pennsylvania, after passengers and crew members fought back against the hijackers.
The attacks, carried out on the morning of September 11, 2001, became one of the deadliest terrorist attacks in history and changed US foreign and domestic policy for decades.
The first strike
At 8.46am, American Airlines Flight 11 crashed into the upper floors of the North Tower of the World Trade Center in New York.
The Boeing 767 had taken off from Boston's Logan International Airport at 7.59am for Los Angeles.
Five al Qaeda hijackers seized control of the aircraft after take-off. There were 92 people aboard the flight — 76 passengers, 11 crew members and five hijackers.
The aircraft struck floors 93 to 99 of the 110-storey North Tower, killing everyone aboard and many people inside the building.
The impact caused severe structural damage and ignited fires fuelled by the aircraft's burning jet fuel.
The second strike
Seventeen minutes later, at 9.03am, the world watched in shock as a second plane struck the South Tower.
United Airlines Flight 175, another Boeing 767 flying from Boston to Los Angeles, had also been hijacked by five terrorists.
There were 65 people aboard — 51 passengers, nine crew members and five hijackers.
The plane struck floors 77 to 85 of the South Tower, causing extensive structural damage and starting intense fires.
Witnesses on the ground recounted the horrifying sight of people leaping to their deaths from the upper floors to escape the unbearable heat and flames.
Although it was hit second, the South Tower collapsed first, at 9.59am, 56 minutes after the impact.
The North Tower collapsed at 10.28am, one hour and 42 minutes after Flight 11 struck it.
The collapses devastated the World Trade Center complex and caused the deaths of thousands of people who had been inside the buildings, as well as first responders who had rushed to the scene.
The impacts had damaged structural columns, while the resulting fires weakened the buildings' structural components.
The combined effects ultimately led to the collapse of both towers.
The Pentagon strike
American Airlines Flight 77 took off from Washington Dulles International Airport at 8.20am, bound for Los Angeles.
The Boeing 757 was carrying 64 people — 53 passengers, six crew members and five hijackers.
The hijacking began between 8.51am and 8.54am, according to the 9/11 Commission.
The aircraft was diverted from its planned route and turned south.
As authorities began to realise that multiple aircraft had been hijacked, Flight 77 disappeared from radar before later being detected heading back towards Washington.
At 9.37am, the hijackers crashed the aircraft into the western side of the Pentagon. Everyone aboard the plane was killed, along with 125 people inside the Pentagon, bringing the death toll from the attack there to 184.
A section of the Pentagon later collapsed at about 10.15am as a result of damage from the impact and the ensuing fire.
The Pennsylvania crash
The fourth hijacked aircraft, United Airlines Flight 93, took off from Newark International Airport in New Jersey at 8.42am, more than 40 minutes after its scheduled departure time.
The Boeing 757 was bound for San Francisco and was carrying 44 people — 33 passengers, seven crew members and four hijackers.
The hijacking began at about 9.28am while the plane was flying over northern Ohio.
By then, information about the attacks in New York was reaching passengers through telephone calls to people on the ground.
At 9.57am, passengers and crew members launched a counterattack against the hijackers in an attempt to regain control of the aircraft.
The struggle continued as the hijackers attempted to maintain control of the plane.
At 10.03am, Flight 93 crashed into an open field near Shanksville in Somerset County, Pennsylvania.
All 44 people aboard were killed.
The 9/11 Commission concluded that the hijackers were heading towards Washington, DC, although the precise intended target was never definitively established.
The US Capitol and White House were among the targets considered by the plot's planners.
The four attacks unfolded in less than two hours, from the first impact at the World Trade Center at 8.46am to the collapse of the North Tower at 10.28am.
The attacks killed 2,977 people from 90 countries: 2,753 in New York, 184 at the Pentagon and 40 aboard Flight 93.
The victims included office workers, airline passengers and crew members, police officers, firefighters, emergency medical workers and other first responders.
The attacks triggered an unprecedented emergency response in the United States and ushered in profound changes in American security, counterterrorism and foreign policy.
Twenty-five years later, September 11 remains one of the defining events of the 21st century.
