The seven-page secret report by the Federal Office for Information Security (BSI), seen by Bild newspaper, discusses five possible ways the NSA could have gained access to Merkel's phone. The story caused outrage in Germany when it came to light in October last year.
Possibilities considered most likely were that US agents either used "passive receiving antenna" planted in central Berlin or else intercepted Merkel's communications as they were transmitted through undersea cables.
The first "very likely" scenario would have involved placing receiving antennas near the capital's Reichstag parliament building and using these to listen to the Chancellor's phone calls and read her text messages.
According to this scenario, potentially any and all mobile communication in the whole of Berlin's central governmental district could have been eavesdropped upon at any time, the report said.
The second "very likely" scenario was that the NSA had tapped undersea cables transmitting communications data in order to listen in specifically on the Chancellor's conversations.
Another less likely, but "probable" NSA tactic would have been to tap and record phone conversations transmitted via microwave link. That would have worked by recording all communications within the area and then filtering individual conversations out.
Alternatively, said the BSI, the NSA may have managed to set up a surveillance system using sensors and other equipment which would have recorded specific telephone conversations automatically.
The least likely scenario, considered "improbable," was that a US agent succeeded in physically getting hold of Merkel's phone and planting a bug on it or otherwise manipulating it, the report said.