Documents from 2001 show the city was concerned about lawsuits over the burning chemicals.
The city assured the public and rescue workers at Ground Zero that the air was safe.
More than 6,000 people have died from 9/11-related diseases.
"They turned around and they lied to us and they hid them. And every mayor for 25 years has been hiding them," Andrew Ansbro with the Uniformed Firefighters Association said.
Critics say the stonewalling needs to end and that the city needs to come clean about air quality in the days, weeks and months after 9/11.
"They made a conscious choice to keep those records hidden instead of studying them and determining what cancers we should be looking for," Ansbro said.
Just blocks from the World Trade Center, leaders in the city council joined advocates for the victims, survivors and their families, urging the Mamdani Administration to release all of the city's 9/11 documents.
"The City of New York has failed to take responsibility for telling the Downtown community and first responders that the air was safe to breathe," Council Speaker Julie Menin said at the news conference.
There are indications that the Giuliani Administration likely knew the air was toxic and was worried about lawsuits.
"There are approximately 35,000 potential plaintiffs, 10,000 would file a claim," his deputy mayor was told in a memo.
It cited health advisories that caused individuals to either return to the area too soon "causing toxic exposure or emotional harm-or too late, causing economic hardship."
They feared rescue workers who "were provided with faulty equipment or no equipment."
Phil Alvarez lost his brother, NYPD detective Lou Alvarez, to 9/11 cancer.
He can't explain why previous mayors have refused to release the records.
"There's no reason not to release this. I'm asking the new mayor to change that pattern so that we can see what's in there," Alvarez said.
"The Department of Investigation is compiling the documents, but needs $3 million to analyze them.
Councilmembers say the mayor needs to provide that money and release every page to the public.
"We really need to see all the possible documents from the city, so that survivors and their families can have some satisfaction - and that's what the full DOI investigation, as required by my law, will give us, and them," City Councilwoman Gale Brewer said in a statement.
The city has previously refused to disclose most environmental records about the toxins, citing potential lawsuits from the thousands of first responders and residents who developed serious health issues that could cost the city billions.
If the DOI gets additional funding, the investigation is expected to be complete by the middle of next year.
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