The US National Security Agency has turned Germany into its most important base for surveillance operations in Europe, developing an “intimate relationship” over the past 13 years, Der Spiegel revealed.

The NSA has not only been spying on German citizens, but the country has also become home to the agency’s key data collection centers in Europe, German magazine Der Spiegel revealed on its website Wednesday in an article based on the analysis of documents leaked by whistleblower Edward Snowden and information from other sources.

“No other country in Europe plays host to a secret NSA surveillance architecture like the one in Germany…In 2007, the NSA claimed to have at least a dozen active collection sites in Germany,” Der Spiegel said.

Snowden’s leaked documents show that the “all-powerful American intelligence agency” has developed an “increasingly intimate relationship with Germany over the past 13 years while massively expanding its presence,” the article reads.

The media reveals that one of the key NSA bases is located in Wiesbaden, southwest Germany, named European Technical Center (ETC), and it has been much developed by the US in the recent years. As the article states Building 4009 of the “Storage Station”, publically known as a US military compound, was never suspected to be a data collecting center.

Snowden’s documents show that following the facility’s refurbishment in 2011, it has been the NSA’s “primary communications hub” in Europe streaming huge amounts of intercepted data to “NSAers, warfighters and foreign partners in Europe, Africa and the Middle East.”

Spiegel also reveals that an “even more powerful and modern facility” is under construction five kilometers from Clay Kaserne at a US military complex in the Erbenheim district of the south-central city of Wiesbaden. Named the “Consolidated Intelligence Center” it is estimated to have cost the US government $124 million.

The European Center for Cryptology (ECC) is yet another key US facility located in the town of Griesheim. It was set up by the US in 2004 and has now become “NSA’s most important outpost in Europe,” according to Spiegel. ECC is an “operative arm” of the NSA’s EU leadership in Stuttgart, says the article citing Snowden’s documents. ECC relies on intelligence gathered through Europe and shared by NSA’s British counterpart GCHQ.

“NSA staff in Griesheim use the most modern equipment available for the analysis of the data streams, using programs like XKeyscore, which allows for the deep penetration of Internet traffic,” the article added.

The agency earlier defended its use of the program as lawful adding that it engages in "extensive, close consultations" with the German government concerning the issue.

The Spiegel article also ponders upon the NSA’s still-close relationship with its German partner, Germany’s foreign intelligence agency the BND (Bundesnachrichtendienst).

“…the exchange of data, spying tools and know-how is much more intense than previously thought,” according to Spiegel. “…yet German officials would seem to know next to nothing about the NSA's activity in their country. For quite some time, it appears, they didn't even want to know. It wasn't until Snowden went public with his knowledge that the German government became active.”

The leaks about the NSA’s surveillance programs were first published in the Guardian and Washington Post in June last year, revealing the extent of US spying on its citizens. Further revelations in February showed that the NSA also spied on world leaders, including the chancellor of Germany. Earlier this June the German Federal Prosecutor opened a long-anticipated investigation into the alleged surveillance of Chancellor Angela Merkel’s phone by the NSA after months of delays.