A windowless skyscraper in the heart of New York City may be the secret Manhattan surveillance hub of the US National Security Agency (NSA), The Intercept has reported. The brutalist, 29-storey AT&T Long Lines Building at 33 Thomas Street has sheer concrete walls that could reportedly survive an atomic explosion; inside, documents leaked by NSA whistleblower Edward Snowden suggest, is one of the agency’s most important spying centres.

Until now, as far as the public was concerned, the building housed a telecommunications exchange routing international phone-calls for AT&T. In 1994, the New York Times reported that 33 Thomas Street was part of the firm’s “giant Worldwide Intelligent Network… directing an average of 175 million phone calls a day.” The Snowden documents, however, describe “a covert monitoring hub… used to tap into phone calls, faxes, and internet data.”

AT&T was exposed by the documents as a willing partner in the NSA’s surveillance operations. After studying architectural plans and interviewing former AT&T employees, The Intercept’s Ryan Gallagher and Henrik Moltke believe the Long Lines Building fits the description of the secret NSA site referred to in the Snowden files as TITANPOINTE. The building, they learned, contains a major so-called “gateway switch” that processes phone-calls to and from the US.