New York is commemorating the 25th anniversary of the September 11, 2001 terror attacks, with thousands expected to gather at the 9/11 Memorial in Lower Manhattan.
The anniversary ceremony will be attended by several US leaders, including Vice President JD Vance and former presidents George W. Bush, Barack Obama, Joe Biden and Bill Clinton.
President Donald Trump will not attend the New York ceremony. Instead, he is expected to pay tribute to the victims at the Pentagon, which was also targeted in the attacks.
No speeches will be delivered during the Ground Zero ceremony. Family members of the victims will read out the names of all 2,977 people killed, while a bell will be rung to mark key moments during the attacks.
The ceremony will also be attended by New York Mayor Zohran Mamdani, the city's first Muslim mayor, despite calls from some conservative figures for him to stay away.
A petition reportedly gathered more than 100,000 signatures urging Mamdani not to attend, while critics questioned his association with people they claimed were dismissive of American institutions or insufficiently critical of extremist ideologies.
However, Mamdani has received praise for recently releasing thousands of documents detailing what city officials knew about air pollution and toxic conditions in Lower Manhattan following the collapse of the World Trade Center.
Mamdani said people became sick because officials they trusted had assured them that the air was safe to breathe.
Nineteen al-Qaeda operatives hijacked four commercial aircraft on September 11, 2001. Two planes were flown into the World Trade Center's Twin Towers, causing the 110-storey buildings to collapse.
A third aircraft struck the Pentagon, while a fourth crashed into a field near Shanksville, Pennsylvania, after passengers attempted to regain control of the plane.
Beyond those killed on the day, more than 9,400 people have since died from illnesses linked to the attacks, according to a federal programme established to provide medical care to those affected.
The anniversary is also a reminder of the lasting impact of the attacks on families and survivors. Some 100 million Americans are now too young to remember the events or were not yet born when they occurred.
Large crowds are expected in Lower Manhattan, with heightened security measures in place around the memorial site.
New York Police Commissioner Jessica Tisch said an anti-drone team would be deployed as a precaution, although authorities said there was no specific intelligence indicating a threat to the ceremony.
Ahead of the anniversary, a drone display featuring 2,977 lights was held over New York Harbor, with each light representing one of the people killed in the attacks.
Al-Qaeda has also marked the anniversary by releasing a video featuring images of former leader Osama bin Laden and his son Hamza, whom Trump said was killed in 2019.