A concrete, windowless fortress in Chicago’s West Loop is one of a handful of buildings in major U.S. cities that serve as spy hubs for the National Security Agency, The Intercept reported.

The online news publication said in its June 25 story that the AT&T building at 10 S. Canal St., which houses telecommunications equipment, is central to a surveillance program that “for years monitored billions of emails, phone calls and online chats passing across U.S. territory.”

Seven other AT&T buildings in cities like New York, Los Angeles and Dallas are also implicated in The Intercept’s investigation, which was based on classified documents, public records and interviews with former AT&T employees.

“Our reporting showed that while (the government) is looking for foreign intelligence, they’re also hovering up Americans’ data,” said Henrik Moltke, an investigative reporter for The Intercept.

The NSA did not respond to our request for comment.

A spokesperson for AT&T said in a statement: “Like all companies, we are required by law to provide information to government and law enforcement entities by complying with court orders, subpoenas, lawful discovery requests and other legal requirements…we ensure that requests for assistance are valid and that we are in compliance with the law.”

The mystery surrounding Chicago’s Canal building is embedded in its very construction, according to the Chicago Architecture Foundation.

Built in the 1970s, at the height of the Cold War, the building was constructed to withstand a possible nuclear attack.

This is how John Augur Holabird, the original architect, described the skyscraper’s purpose:

“In case the atomic bomb hits Milwaukee, you’d be happy to know your phone lines will still go through, even though the rest of us are wiped out and that’s what that building as for,” Holabird said in a 1998 interview.

In a city celebrated for its architecture, the building is forgettable and that’s by design, said CAF Director of Interpretation Adam Rubin.

The building is window-less on most of its 28 floors and insulated with thick, concrete walls. It originally had its own generator and water well, so that telecommunication systems would be protected in the event of war, Rubin said.

“This building was built to look as inconspicuous as possible, so you look at it, take a moment to notice it has no windows, then forget about it and keep moving,” Rubin said.