German prosecutors have dropped an investigation into suspected mass phone tapping of German citizens by British and US spies, Reuters reports. No concrete evidence of any criminal activity has been found, they said on Thursday. The spy scandal had been triggered by revelations by former US National Security Agency (NSA) contractor Edward Snowden. The case has strained ties between Berlin and Washington. Although the techniques used in US surveillance were known to German intelligence agencies, there was no evidence of them being used to target Germany, prosecutors in Karlsruhe said, explaining why they will no longer pursue the case.