New York (AFP)
New York commemorates the 25th anniversary of the devastating September 11, 2001 attacks on Friday with a ceremony marked by US President Donald Trump's absence.
Trump will pay his respects at the Pentagon, not New York, where hijacked planes destroyed the World Trade Center and killed most of the 2,977 total victims on that world-altering day.
Vice President JD Vance and Trump's four living predecessors -- Republican George W. Bush, who was in office during the attacks, and Democrats Joe Biden, Barack Obama and Bill Clinton -- are all expected at the "Ground Zero" site in lower Manhattan.
No one will give speeches at the ceremony, while bereaved family members will read out the names of all of those killed and a bell rung to mark major moments during the attack.
Alongside the former presidents will be Democratic Mayor Zohran Mamdani, the Big Apple's first Muslim mayor.
Mamdani won plaudits however for recently revealing thousands of documents showing what city officials knew about toxicity levels of the air in Lower Manhattan following the attacks.
After the initial dust cloud that followed the collapse of the World Trade Center, months of pollution and lingering dust ensued, causing cancers from which thousands of New Yorkers still suffer today.
'Toxic air'
"People got sick because the leaders they trusted lied and told them they were safe to breathe in toxic air," Mamdani said on Tuesday.
Nineteen Al Qaeda operatives hijacked four planes on that nearly cloudless day in 2001. Two jets slammed into the World Trade Center's Twin Towers, causing the iconic 110-story buildings to collapse.
A third plane struck the Pentagon, while a fourth crashed into a field near Shanksville, Pennsylvania, after passengers fought back.
Beyond those killed on September 11, more than 9,400 other people have died from 9/11-related illnesses, according to the federal program established to provide medical care to those affected.
Some 100 million Americans were not even born or are too young to remember the tragic events.
Huge crowds are expected to gather in Lower Manhattan for the memorial service.
The Secret Service has erected a security ring of steel around the site of the 9/11 Memorial and Museum.
Police commissioner Jessica Tisch said an anti-drone team would be deployed as a precaution, but stressed there was no intelligence of a specific threat to the gathering.
Ahead of the memorial, a drone show with 2,977 lights rose above New York Harbor, one for each person killed in the attacks.