Toxic air quality was being recorded in Manhattan soon after the 9/11 attacks, even as officials played down the health risks to the public, according to a trove of newly released files.

New York City Mayor Zohran Mamdani said the 170,000 documents show asbestos was found "nearly everywhere" they tested near Ground Zero, the site where the World Trade Center towers collapsed in 2001.

"People got sick because the leaders they trusted lied and told them they were safe to breathe in toxic air," Mamdani said at a news conference with survivors.

Illnesses related to the dust and smoke from the collapse of the Twin Towers have claimed more lives than the attacks themselves, US health officials say.

The document release resolves a lawsuit brought by advocacy group 9/11 Health Watch, which sued the city for the documents.

City officials found 68 boxes of the documents last year. Officials had previously said they did not have them.

"Administration after administration has ignored those calling for answers and clarity," Mamadani said, standing next to television host and 9/11 survivor advocate Jon Stewart. "We will do no such thing."

Thousands of people have experienced health effects like cancer from the terrorist attacks, including many who have died, after breathing in hazardous contaminants from the building collapses.

The pages of air quality reports and other documents were released through an online portal, timed with the 25th anniversary of the attacks.

Mamdani said City Hall will spend more than $34m (£25m) on the effort. He said the documents show what the city "knew about the toxic air around Ground Zero, when they knew it, and how they acted to limit liability".

More documents are to be shared over the course of the next year.

The release comes after years of survivors advocating for more transparency from city officials.

Speaking at the news conference on Tuesday, Stewart said the documents show the city didn't recognise that the air that everyone was breathing in downtown Manhattan was toxic until October of 2001.

"So to be fair to them, they only buried that information for 25 years," he said sarcastically.

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.

The document states that the city could be sued because health advisories caused individuals "to return to the area too soon (causing toxic exposure or emotional harm)".

Days after the attack, city and federal officials – including the US Environmental Protection Agency (EPA) – declared that the air near the Twin Towers was safe, though research suggests people were breathing in toxic chemicals.

Christine Todd Whitman, the head of the EPA at the time, later apologised for this declaration, saying the agency "did the very best we could at the time with the knowledge we had".

Elizabeth Cascio, a former New York Fire Department (FDNY) emergency medical technician, previously told the BBC that she suffered sinus issues and headaches after responding to the attacks.

"We all knew the air quality was not safe – it was very toxic in terms of how it felt," she said. "Initially, when I came off the bus and arrived at the Trade Center, you felt like you had to hold your breath. But there's only so long you can hold your breath. I could feel the particles coming into my nose and mouth and thought: 'This can't be good'."