It took less than two hours for an ordinary September morning in the United States to become one of the darkest days in the country's history.
On September 11, 2001, 19 terrorists linked to al Qaeda seized four commercial aircraft and carried out a coordinated attack that left 2,977 people dead and changed the course of American security and foreign policy for decades.
The first two planes were flown into New York's World Trade Center, bringing down the Twin Towers and killing thousands inside and around the complex.
A third aircraft was deliberately crashed into the Pentagon in Arlington, Virginia, while passengers and crew aboard the fourth plane fought the hijackers before it came down in a Pennsylvania field.
The attacks began shortly before 9am and unfolded with such speed that authorities were forced to respond to several hijacked aircraft at the same time.
Flight 11 begins the attack
At 8.46am, American Airlines Flight 11 struck the North Tower of the World Trade Center in New York.
The Boeing 767 had departed Boston's Logan International Airport at 7.59am on a scheduled flight to Los Angeles. There were 92 people aboard — 76 passengers, 11 crew members and five hijackers.
Five al Qaeda hijackers had taken control of the aircraft after take-off and directed it into floors 93 to 99 of the 110-storey North Tower.
The impact killed everyone on the aircraft and many people inside the tower. It also caused major structural damage and started fires as the aircraft's burning jet fuel spread through the building.
Twin Towers of the World Trade Center in New York City on September 11, 2001.
PHOTO/HANDOUT
A second plane follows
The scale of the disaster became clear just 17 minutes later.
At 9.03am, United Airlines Flight 175 crashed into the South Tower, after being hijacked while travelling from Boston to Los Angeles.
The Boeing 767 was carrying 65 people — 51 passengers, nine crew members and five hijackers.
The aircraft hit floors 77 to 85, tearing into the tower and triggering powerful fires.
On the ground, witnesses watched in horror as people trapped in the upper sections of the towers tried desperately to escape the heat and flames, with some jumping from the buildings.
The South Tower, despite being struck after the North Tower, collapsed first at 9.59am, 56 minutes after Flight 175 hit it.
The North Tower came down at 10.28am, one hour and 42 minutes after Flight 11 crashed into it.
The collapses devastated the World Trade Center complex and claimed the lives of thousands of people inside the towers and others who had arrived to assist, including police officers, firefighters and emergency workers.
The aircraft impacts damaged the towers' structural columns, while the fires weakened other parts of the buildings. The combined damage eventually caused both towers to collapse.
Attack reaches the Pentagon
The third hijacked plane was heading towards Washington as the attacks in New York unfolded.
American Airlines Flight 77 left Washington Dulles International Airport at 8.20am for Los Angeles with 64 people aboard — 53 passengers, six crew members and five hijackers.
The 9/11 Commission established that the hijacking began between 8.51am and 8.54am.
The aircraft was taken off its original route and turned south. As officials tried to understand what was happening to several aircraft, Flight 77 disappeared from radar before it was later detected travelling back towards Washington.
At 9.37am, the Boeing 757 was deliberately crashed into the western side of the Pentagon, the headquarters of the US Department of Defense.
All 64 people aboard the aircraft died, while 125 people inside the Pentagon were also killed. The attack there claimed 184 lives in total.
At about 10.15am, part of the Pentagon collapsed after suffering damage from the aircraft impact and the fire that followed.
Flight 93 becomes the final target
United Airlines Flight 93 was the last of the four hijacked aircraft to leave the ground.
It departed Newark International Airport in New Jersey at 8.42am, more than 40 minutes later than scheduled, and was headed for San Francisco.
The Boeing 757 had 44 people aboard — 33 passengers, seven crew members and four hijackers.
The hijacking started at about 9.28am as the plane flew over northern Ohio.
By that point, passengers were beginning to learn about the attacks in New York through phone calls with people on the ground.
At 9.57am, several passengers and crew members launched an effort to overpower the hijackers and retake the aircraft.
The confrontation continued as the hijackers tried to keep control of the plane.
Six minutes later, at 10.03am, Flight 93 crashed into an open field near Shanksville in Somerset County, Pennsylvania. All 44 people aboard died.
The 9/11 Commission later concluded that the aircraft was heading towards Washington, DC, although its exact intended target was never confirmed.
The White House and US Capitol were among the locations considered by the plot's planners.
The human cost
From the first impact at 8.46am to the collapse of the North Tower at 10.28am, the attacks unfolded in less than two hours.
A total of 2,977 people from 90 countries died.
New York accounted for 2,753 deaths, while 184 people were killed in the Pentagon attack and 40 people aboard Flight 93 lost their lives.
The victims came from many walks of life. They included office workers, airline passengers and crew, police officers, firefighters, emergency medical workers and other first responders.
The attacks prompted an unprecedented emergency operation across the United States and led to lasting changes in American security, counterterrorism efforts and foreign policy.
Twenty-five years on, the events of September 11 remain a defining moment of the 21st century, remembered not only for the destruction and loss of life but also for the far-reaching changes that followed.