NEW YORK (AFP)

New York Mayor Zohran Mamdani announced on Tuesday the release of tens of thousands of documents showing the city did not disclose all it knew about air toxicity after 9/11.

The release comes ahead of the 25th anniversary of the September 11, 2001, attacks that killed 2,977 people in the United States, mostly in the collapse of the Twin Towers in New York.

In the years since, more than 9,400 have died from illnesses linked to their exposure to the area, according to a federal medical monitoring programme.

"These are documents that show what the city knew about the toxic air around Ground Zero, when they knew it, and how they acted to limit liability," the mayor told a news conference following the online release of 170,000 pages of documents on health concerns, air quality, and the city's response to the attacks.

The release is part of an agreement with 9/11 Health Watch, a victims' advocacy group that had taken legal action to compel the city to make the documents public.

"For 25 years, four different Mayoral administrations kept from the public, the US Congress, and the City Council documents showing what the City actually knew about the danger of the toxic chemicals in Lower Manhattan and Western Brooklyn after the World Trade Centre collapse, even as City officials continued messaging to the public that the air was 'safe and acceptable'," the group's director Benjamin Chevat said in a statement.

After the initial dust cloud that followed the collapse of the World Trade Centre, months of pollution and lingering dust ensued, causing cancers that thousands of New Yorkers still suffer today.

When asked about the names of officials who had withheld information from the public, Mamdani said, "this goes across a multitude of those that New Yorkers have trusted. People got sick because the leaders they trusted lied and told them they were safe to breathe in toxic air."

Roughly 173,000 documents released Tuesday, and additional ones which will be published online in waves over the next 12 months, could implicate two former New York mayors. They are Rudy Giuliani, in office at the time of the attacks, and then Michael Bloomberg, mayor from January 2002 to December 2013.

On September 11, 2001, hijackers seized control of two planes that struck the two World Trade Centre towers. A third hijacked jet smashed into the Pentagon, and a fourth, which seemed to be headed for the Capitol or the White House, crashed in woodland near Shanksville, Pennsylvania, after passengers fought back and seized control.