Newly declassified intelligence draws attention
The 25th anniversary has also brought renewed attention to intelligence available to US authorities before the attacks. The CIA released dozens of previously classified presidential daily briefs and related material concerning Osama bin Laden and al-Qaeda.
The documents provide additional insight into what US intelligence agencies knew about the terrorist organisation and its activities before September 11.
The long-term impact of 9/11
The attacks fundamentally changed US national security policy. Washington launched the war in Afghanistan shortly afterwards and expanded counterterrorism operations around the world.
The attacks also led to major changes in airport security, intelligence coordination, surveillance and immigration policies in the United States.
A legacy that continues
The consequences of September 11 continue to affect American politics, foreign policy and national security more than two decades later.
Thousands of first responders and others who were exposed to toxic dust and debris at the World Trade Center site have also suffered serious health consequences in the years since the attacks.
As the United States marks 25 years since September 11, ceremonies across the country are once again focused on remembrance, the families who lost loved ones and the lasting impact of one of the most consequential events in modern American history.
