New York City officials had evidence of possibly dangerous air conditions in Lower Manhattan after the Sept. 11, 2001, terrorist attacks even as the public was repeatedly reassured that returning to the area posed little long-term health risk, newly discovered documents suggest.

The documents, recently unearthed by Mayor Zohran Mamdani’s administration and reviewed by the New York Times, provide new details about the city’s monitoring of the toxic dust and debris that blanketed the area after the World Trade Center collapsed.