What happened on 9/11 and who is Khalid Sheikh Mohammed?

On Tuesday 11 September 2001, suicide attackers seized US passenger jets and crashed them into two New York skyscrapers, killing thousands of people.

Twenty-three years later, Khalid Sheikh Mohammed and two other men accused of plotting the attack - known as 9/11 - are expected to plead guilty in a military court.

What unfolded that day has had profound consequences across the globe.

Four planes flying over the eastern US were seized simultaneously by small teams of hijackers.

They were then used as giant, guided missiles to crash into landmark buildings in New York and Washington.

Two planes hit the Twin Towers of the World Trade Center in New York, with the second strike 17 minutes after the first.

The buildings were set on fire, trapping people on the upper floors, and enveloping the city in smoke. In less than two hours, both 110-storey towers collapsed in massive clouds of dust.

A third plane destroyed the western face of the Pentagon - the giant headquarters of the US military just outside the nation's capital, Washington DC.

The fourth plane crashed in a field in Pennsylvania after passengers fought back. It is thought the hijackers had meant to attack Washington's Capitol Building.

The youngest victim was two-year-old Christine Lee Hanson and the oldest was 82-year-old Robert Norton, who were both passengers on planes.

When the first plane struck, an estimated 17,400 people were in the towers. Nobody survived above the impact zone in the North Tower, but 18 managed to escape from the floors above the impact zone in the South Tower.

Citizens of 77 different countries were among the casualties. New York City lost 441 first responders.

Thousands of people were injured or later developed illnesses connected to the attacks, including firefighters who had worked in toxic debris.

An Islamist extremist network called al-Qaeda planned the attacks from Afghanistan.

Led by Osama Bin Laden, al-Qaeda blamed the US and its allies for conflicts in the Muslim world.

Nineteen people carried out the hijackings, working in three teams of five and one of four (on the plane which crashed in Pennsylvania).

Each group included someone who had received pilot training. This was carried out at flying schools in the US itself.

Fifteen hijackers were Saudis like Bin Laden. Two were from the United Arab Emirates, one was from Egypt and one was from Lebanon.

Less than a month after the attacks, President George W Bush led an invasion of Afghanistan - supported by an international coalition - to eradicate al-Qaeda and hunt down Bin Laden.

However, it was not until 2011 that US troops finally located and killed Bin Laden in neighbouring Pakistan.

The alleged planner of the 9/11 attacks, Khalid Sheikh Mohammad, was arrested in Pakistan in 2003.

Al-Qaeda still exists. It is strongest in Sub-Saharan Africa but even now has members inside Afghanistan.

US troops left Afghanistan in 2022 after nearly 20 years, stoking fears from many that the Islamist network could make a comeback.

He is considered by prosecutors to be the 9/11 mastermind, taking his hijacked planes idea to Bin Laden and later helping to recruit the pilots.

Born in Kuwait, he studied engineering in the US before fighting in Afghanistan in the 1980s.

Even before 9/11, the FBI were on his trail as they believed him to be behind other bombings and thwarted attacks.

In 2006, three years after being captured, he was sent to the US Navy base at Guantanamo Bay, Cuba, and has been held there since.

One reason he was not put on trial during that period was the fear that the brutal interrogation techniques used on him could have undermined the prosecution's case.

Now he and two others held there have agreed to plead guilty to the murder of the 2,976 people listed in the charge sheet. In exchange they will not receive the death penalty.

The other two men are Walid Muhammad Salih Mubarak Bin Attash and Mustafa Ahmed Adam al-Hawsawi.

Some relatives of those killed on 9/11 are deeply unhappy about the plea deal, describing it as a victory for the conspirators.

Flight safety was tightened around the world in the years following the 9/11 attacks.

In the US, the Transportation Security Administration (TSA) was created to beef up security at airports and on planes.

It took more than eight months to clean up "Ground Zero" - the site of the fallen Twin Towers.

A memorial and a museum now stands on the site, and buildings have risen up again, to a different design.

The completed centrepiece - One World Trade Center, or "Freedom Tower" - stands even higher (1,776ft (541m) than the original North Tower, which was 1,368ft.

Reconstruction at the Pentagon took just under a year, with staff back in their offices by August 2002.