It's a prime site, a diplomat's dream. Is there any better location for an embassy than Berlin's Pariser Platz? It's just a few paces from here to the Reichstag. When the American ambassador steps out the door, he looks directly onto the Brandenburg Gate.

When the United States moved into the massive embassy building in 2008, they threw a huge party. Over 4,500 guests were invited. Former President George H. W. Bush cut the red-white-and-blue ribbon. Chancellor Angela Merkel offered warm words for the occasion. Since then, when the US ambassador receives high-ranking visitors, they often take a stroll out to the roof terrace, which offers a breathtaking view of the Reichstag and Tiergarten park. Even the Chancellery can be glimpsed. This is the political heart of the republic, where billion-euro budgets are negotiated, laws are formulated and soldiers are sent to war. It's an ideal location for diplomats -- and for spies.

Research by SPIEGEL reporters in Berlin and Washington, talks with intelligence officials and the evaluation of internal documents of the US' National Security Agency and other information, most of which comes from the archive of former NSA contractor Edward Snowden, lead to the conclusion that the US diplomatic mission in the German capital has not merely been promoting German-American friendship. On the contrary, it is a nest of espionage. From the roof of the embassy, a special unit of the CIA and NSA can apparently monitor a large part of cell phone communication in the government quarter. And there is evidence that agents based at Pariser Platz recently targeted the cell phone that Merkel uses the most.

The NSA spying scandal has thus reached a new level, becoming a serious threat to the trans-Atlantic partnership. The mere suspicion that one of Merkel's cell phones was being monitored by the NSA has led in the past week to serious tensions between Berlin and Washington.