At a ceremony at the 9/11 memorial in New York, which was attended by Mayor Eric Adams and FBI Director Kash Patel, the names of the victims were read out.
On Thursday evening, numerous buildings in New York were set to glow blue and two blue pillars of light were to rise into the sky above the city, near the site where the World Trade Center towers once stood.
US President Donald Trump and First Lady Melania Trump appeared at a memorial meeting at the Pentagon.
On September 11, 2001, about 3000 people were killed in attacks that shocked the world.
Islamist militants steered three hijacked airliners into the twin towers of the World Trade Center in New York and into the Pentagon in Washington DC.
A fourth plane crashed in Pennsylvania.
It remains unclear what the hijackers' target was with the fourth plane.
A memorial service was held in in Shanksville, Pennsylvania on Thursday to remember the victims.
The United States entered the "war on terror" after the 9/11 attacks.
The invasion of Afghanistan was followed by the war in Iraq in 2003, the effects of which ultimately resulted in the founding of the Islamic State group.
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