Spyscraper? This is the windowless AT&T Long Lines Building in New York. It's an AT&T property but has been linked to the NSA's Titanpointe data spying operation

Even in skyscraper-filled lower Manhattan, the windowless mass of 33 Thomas Street stands out: A faceless concrete tower, topped with vents and impervious to the outside world.

Constructed and owned by AT&T, and known as the 'Long Lines Building,' it's officially a telecommunications hub with its own power and water, capable of sheltering those inside against two weeks of nuclear fallout.

But documents leaked by Edward Snowden, along with others gathered by The Intercept, suggest that it has a worrying secret identity: An NSA surveillance base named Titanpointe that spies on phone calls, fax messages and internet data.

The building, which was developed under the name 'Project X,' opened in 1974 about 14 blocks north of Wall Street and houses an international 'gateway switch,' an AT&T employee told The Intercept.

That switch is a major route by which international calls and data enter the US telecomms system - meaning that if someone has access to the data, they could spy on world communications.

If that's the case then it would be a vital location for the NSA's controversial surveillance program, which has allegedly spied on at least 38 countries, including allies.

The program has reportedly also spied on communications to and from the UN, the International Monetary Fund and the World Bank.

Top-secret NSA documents indicate that the NSA has indeed set up a site, codenamed 'Titanpointe,' in the building, The Intercept said.

Faceless: The building is home to an AT&T hub that routes data between the US and the rest of the world. Leaked documents suggest the NSA is spying on that data

Feds: It's located two blocks from an FBI field office. A field guide tells Titanpointe visitors to take an unmarked car from the FBI before visiting; the base's location is not made explicit

Satellite: The AT&T building has a satellite dish on it; Titanpointe has been linked to Skidrowe, which intercepts satellite data. It also has parking spaces marked 'AWM' - code for the feds

NSA guides obtained by The Intercept and documentary unit Field of Vision, tell agents visiting Titanpointe that it is located in New York and direct them to the FBI's field office in the city.

THE CODE NAMES A number of code names have been linked to the AT&T Long Lines Building: AWM: The building has parking spaces marked 'AWM' - a code used to denote federal agencies. BLARNEY: A program developed in the early 1970s to mass-collect content and metadata from communications. This was done with 'commercial partners' - companies such as AT&T. LITHIUM: The NSA codename for AT&T, which owns the Long Lines Building in downtown Manhattan, New York City. PROJECT X: Not an NSA code name, but the working title for AT&T's Long Lines Building, which houses a telecomms switch connecting the US with the rest of the world. It is also designed to resist atomic blasts and has its own water, food and electricity supplies. SKIDROWE: An NSA program designed to intercept satellite data including emails, chats, Skype calls, passwords, and internet browsing histories. TITANPOINTE: A site for the NSA's mass data collection program, confirmed to be in New York and believed to be located in the Long Lines Building. Advertisement

There they can pick up 'cover vehicles' to travel to the site, which they will be shown around by representatives of Lithium - the NSA's codename for AT&T.

The FBI field office is located two blocks from the Long Lines Building. There are parking bays outside the windowless skyscraper marked 'AWM,' a code denoting federal agencies.

The building also has a satellite dish, which is notable because the NSA documents link Titanpointe to the Skidrowe program, which intercepts satellite data including emails, chats, Skype calls, passwords, and internet browsing histories.

Snowden-leaked documents also link Titanpointe to the Blarney program, which was developed in the early 1970s to take, in mass, communications content (such as conversations) and metadata (such as call times).

That data was collected using 'commercial partnerships' with companies such as AT&T.

According to a leaked 2012 email, a 'collection against the email address of the UN General leading the monitoring mission in Syria,' was performed by the NSA's Blarney operatives against the UN's New York mission.

That operation most likely involved Titanpointe - and therefore the Long Lines Building - The Intercept said.

'Such spying activities are totally unacceptable breaches of trust in international cooperation,' Mogens Lykketoft, former president of the UN's general assembly, said.

AT&T itself has a long history of aiding US spy networks. It was identified in 2015 by The New York Times as the NSA's most enthusiastic telecomms helper, providing them with access to billions of emails.

Elizabeth Goitein, co-director of the liberty and national security program at the Brennan Center for Justice, told The Intercept she was concerned about what the documents showed.

'This is yet more proof that our communications service providers have become, whether willingly or unwillingly, an arm of the surveillance state,' she said.

'The NSA is presumably operating under authorities that enable it to target foreigners, but the fact that it is so deeply embedded in our domestic communications infrastructure should tip people off that the effects of this kind of surveillance cannot be neatly limited to non-Americans.'

Rooms without a view: The Long Lines Building was made to withstand an atom bomb and has its own power and water supply - but no windows higher than the lobby (pictured)

AT&T spokesman Fletcher Cook said the company did not 'allow any government agency to connect directly to or otherwise control our network to obtain our customers' information.

'Rather, we simply respond to government requests for information pursuant to court orders or other mandatory process and, in rare cases, on a legal and voluntary basis when a person's life is in danger and time is of the essence, like in a kidnapping situation.'

He added that NSA operatives 'do not have access to any secure room or space within our owned portion of the 33 Thomas Street building.'

When asked about the four floors of the building once owned by Verizon and still maintained under an easement - the building is owned by AT&T - he declined to comment.

Leaked documents suggest that NSA equipment at Titanpointe is contained within a separate secure room, connected to the server rooms of its 'partner company'.

The NSA documents do not imply that Verizon is connected to Titanpointe, although a technician who did not wish to be named said that there are still a few Verizon employees based there.

Both the NSA and Verizon declined to comment to The Intercept.