This September marks 25 years since the September 11 terrorist attacks, and a recently uncovered memo is shedding new light on what New York City officials may have known about air quality risks at ground zero in the weeks following the disaster.

In the months after the Twin Towers fell, thousands of volunteers, first responders, office workers and students returned to lower Manhattan. Many helped sift through smoldering debris, while others resumed daily life near the World Trade Center site. At the time, officials assured the public that the air was safe to breathe.

But a memo sent just one month after the attacks suggests that, behind the scenes, there were concerns.

The document warns that as many as 35,000 plaintiffs could sue the city in connection with events following 9/11. One potential reason cited was that “health advisories caused individuals either to return to the area too soon causing toxic exposure or emotional harm, or too late causing economic hardship.”

The memo was uncovered by attorneys Matthew McCauley and Andrew Carboy. McCauley, an attorney with 9/11 Health Watch, is also a former member of the NYPD and 9/11 responder.

“They knew there were concerns that were there,” McCauley said.

He argues the document indicates city officials were aware that the air posed potential dangers, even as people—including children—were sent back to nearby schools and workplaces.

“To send people back and to send kids back to school in those locations—that’s where things get a little questionable,” McCauley said. “And if you know the air wasn’t safe at that point, there are other things that could have been done.”

According to advocacy groups, more than 9,000 deaths have been linked to toxins released after 9/11, and roughly 49,000 people have been diagnosed with cancers connected to exposure at or near ground zero. Some believe those numbers could have been significantly lower if clearer warnings had been issued earlier.

“We could have spared tens of thousands of people who got sick and many who have died,” said John Feal of the Feal Good Foundation. “We could have spared the illnesses, the cancers, the COPD, the pulmonary fibrosis.”

News 12 has reached out to the current New York City administration for comment, as well as to the individual who received the memo 25 years ago. As of publication, no responses have been received.