New York Mayor Zohran Mamdani announced on Tuesday the release of tens of thousands of documents showing that the city did not disclose everything 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 from the collapse of the Twin Towers in New York.

More than 9,400 have died from illnesses linked to their exposure to the area in the years since, according to a federal medical monitoring program.

Thousands of people have suffered health problems, including cancer, as a result of the terrorist attacks, with many dying after inhaling hazardous contaminants released by the collapse of the buildings, according to the BBC.

“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 sued the city to obtain the documents.

The BBC reported that city officials found 68 boxes of the documents last year. Officials had previously said they did not have them.

“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 Center 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.

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.”

“Administration after administration has ignored those calling for answers and clarity,” BBC quoted Mamdani as saying, as he stood next to television host and 9/11 survivor advocate Jon Stewart.

“We will do no such thing.”

The documents include an October 2001 file known as the “Harding Memo”, in which an aide warned former Deputy Mayor Robert Harding — serving under Mayor Rudy Giuliani — that the city could face as many as 35,000 legal claims for its response to the attacks, BBC reported.

The document states that the city could be sued because health advisories had caused individuals “to return to the area too soon (causing toxic exposure or emotional harm)”, the BBC reported, quoting the document.

Two weeks ago, a US Air Force judge ordered Khalid Sheikh Mohammed, the accused mastermind of the 9/11 attacks, and three co-defendants to go on trial before a military tribunal in June 2028.

Khalid remains the most well-known inmate at the Guantanamo detention facility, which was set up in 2002 by then-US president George W. Bush to house foreign terrorist suspects following the September 11, 2001 attacks.

He is accused of orchestrating the plot to fly hijacked passenger jetliners into the World Trade Center in New York City and into the Pentagon.