Twenty-five years later, one question still refuses to go away: if there were warnings, why wasn't anyone able to stop it?

On September 11, 2001, four commercial aircraft were hijacked in a coordinated terrorist operation that changed the course of American history and reshaped global security. Two planes struck the World Trade Center in New York, one hit the Pentagon and another crashed near Shanksville, Pennsylvania, after passengers and crew fought back.

A total of 2,977 people were killed, excluding the 19 attackers.

But some of the most revealing details of the 9/11 attacks happened before the first plane hit.

There were years of terrorist activity, intelligence reports, suspicious movements and warnings. Some of the men involved had even triggered airport security systems.

So, what exactly happened? And what did the world miss?

9/11 Attacks: The Story Began Years Before September 11

The plot did not emerge overnight.

Al Qaeda had already carried out or supported attacks against US interests, including the 1993 World Trade Center bombing, the 1998 US embassy bombings in Kenya and Tanzania, and the 2000 USS Cole attack.

The 9/11 Commission identified Khalid Sheikh Mohammed (KSM) as the principal architect of the operation. Its report traces how the idea of using aircraft as weapons developed into what it called the “planes operation.”

The FBI says all 19 men who carried out the attacks had been trained by al Qaeda. Osama bin Laden, the organisation's leader, was ultimately identified as the person responsible for orchestrating the attacks. He remained at large for almost a decade before being killed in Pakistan in 2011.

 

Who Were the ‘Muscle Hijackers’?

The 19 men involved were not all trained for the same role.

Four were the pilot-hijackers: Mohamed Atta, Marwan al-Shehhi, Ziad Jarrah and Hani Hanjour.

The remaining 15 were described by the 9/11 Commission as “muscle hijackers.” Their job was to overpower passengers and crew and help seize control of the aircraft.

The group also spent months preparing inside the United States. According to the FBI, the men began entering the country in January 2000, and all 19 were there by early July 2001.

The operation required much more than taking over four planes. It involved travel, money, accommodation, flight training and coordination across different countries and cities.

That level of preparation is one reason the plot remains such an important case study in counterterrorism.

Some Hijackers Had Already Triggered Airport Security Alerts

This is one of the lesser-known details that can easily get lost in the larger story.

Mohamed Atta was selected by the airport's passenger-prescreening system before flying from Portland, Maine, to Boston. Three other members of the Flight 11 team were also selected in Boston.

But the security procedures of 2001 were very different from those people encounter today. Being selected largely meant additional baggage handling rather than the extensive passenger screening now associated with air travel.

At Washington Dulles, several members of the Flight 77 team also triggered security alarms. Khalid al-Mihdhar and Majed Moqed set off metal detectors, while Nawaf al-Hazmi triggered alarms, was hand-wanded and had his carry-on checked for explosives.

Yet they still boarded their aircraft.

The crucial point is that these alerts did not mean airport officials knew they were terrorists. The systems detected individual problems but were not designed to connect those incidents to a wider terrorist plot.

The Warnings Before the 9/11 Attacks

This is perhaps the biggest question surrounding the tragedy.

US intelligence agencies already knew about Osama bin Laden and the threat posed by his organisation. During the summer of 2001, warnings became increasingly serious.

The 9/11 Commission titled its chapter on this period “The System Was Blinking Red.”

Its report records a stream of warnings about a possible major al-Qaeda operation. On July 5, senior officials were warned that a significant terrorist attack was imminent and could occur at any time. Later that month, intelligence officials described the threat as exceptionally serious.

Yet the warning information remained fragmented.

Individual cases were being handled by different agencies, while no one successfully connected them to the broader threat picture.

The Commission concluded that failures of imagination, policy, capabilities and management contributed to the inability to prevent the attack.

In simple terms: the pieces of the puzzle existed, but they were not put together in time.

What Happened on September 11?

The attacks unfolded in less than two hours.

• 8:46 am: American Airlines Flight 11 struck the North Tower.
• 9:03 am: United Airlines Flight 175 struck the South Tower.
• 9:37 am: American Airlines Flight 77 hit the Pentagon.
• 9:59 am: The South Tower collapsed.
• 10:03 am: United Airlines Flight 93 crashed near Shanksville, Pennsylvania.
• 10:28 am: The North Tower collapsed.

The second plane hitting the South Tower made it clear that the first crash was not an accident but part of a coordinated attack.

The Federal Aviation Administration also ordered civilian aircraft across the United States to land, bringing commercial air traffic to an unprecedented halt.

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Flight 93 and the Passengers Who Fought Back

Flight 93 became a story of resistance amid an otherwise devastating morning.

Passengers and crew members learned about the attacks through phone calls and began to understand that their aircraft was likely being used as a weapon.

They decided to fight back.

The aircraft crashed into a field near Shanksville before reaching its intended target. The 9/11 Commission concluded that the passengers' actions prevented the hijackers from reaching their objective.

Their actions remain one of the defining human stories of that day.

The Impact Didn't End When the Towers Fell

The immediate death toll tells only part of the story.

First responders, rescue workers, recovery workers, survivors and people who lived or worked around Ground Zero were exposed to dust, smoke and other hazardous substances.

According to the CDC, more than 96,000 people have been diagnosed with physical or mental health conditions linked to exposure to the attacks and their aftermath. The World Trade Center Health Program now serves nearly 150,000 members.

The James Zadroga 9/11 Health and Compensation Act of 2010 established the federal health programme that continues to provide monitoring and treatment for affected responders and survivors.

For thousands of people, the consequences therefore continued long after September 11, 2001.

How 9/11 Changed Security, Travel and Government

The attacks transformed American national security.

The US launched military operations in Afghanistan in October 2001 as part of its response to al-Qaeda and the Taliban's provision of sanctuary to the organisation.

At home, the USA PATRIOT Act expanded government powers related to surveillance and counterterrorism investigations.

The Department of Homeland Security was subsequently created, while the Transportation Security Administration (TSA) became central to the new aviation-security system.

For travellers, stricter screening, baggage checks and cockpit-security measures became part of everyday life.

But the new security regime also created a difficult question: how much privacy and individual freedom should governments be allowed to sacrifice in the name of preventing terrorism?

That debate continues 25 years later.

Did the World Actually Learn the Lessons?

The answer is complicated.

Intelligence-sharing improved. New counterterrorism structures were created. Agencies became more focused on identifying and preventing terrorist plots before they could reach the operational stage.

But major intelligence failures continued.

The flawed intelligence surrounding Iraq's alleged weapons of mass destruction became another example of the danger of poor intelligence assessment and strategic decision-making.

Meanwhile, the threat landscape changed. Counterterrorism agencies increasingly had to contend with lone attackers, online radicalisation, domestic extremism and politically motivated violence.

At the same time, Western governments increasingly faced strategic challenges from countries such as Russia and China.

The lesson may therefore be bigger than simply “watch for terrorists.”

Modern intelligence agencies can collect enormous amounts of information. The harder question is often how to identify what matters, connect seemingly unrelated signals and make the right decision before it is too late.

25 Years Later, Why Does 9/11 Still Matter?

The legacy of the 9/11 attacks can be seen in places most people encounter every day - from airport security and surveillance to immigration, counterterrorism and emergency preparedness.

But perhaps the most important legacy is human.

Nearly 3,000 people lost their lives that morning. Thousands more carried physical and psychological scars into the decades that followed.

Twenty-five years later, remembering 9/11 should therefore mean more than remembering the images of collapsing towers.

It should mean remembering the warnings that were missed, the intelligence that was not connected, the people who fought back, the lives that were lost and the decisions that changed the world that followed.

Because the most uncomfortable lesson of September 11 may be this:

A warning can exist. Information can exist. People can even see pieces of the truth - and catastrophe can still happen when nobody sees the whole picture.