Prosecutors in Germany have dropped an investigation into alleged hacking of information from German citizens and officials by the US National Security Agency (NSA) and its British partner.

The prosecution service said Thursday that the charges against the NSA were dropped due to lack of concrete evidence that could prove the intelligence service and its British partner had been involved in mass Internet and phone data surveillance in Germany as reports claimed before the opening of the probe four years ago.

Edward Snowden, a fugitive US intelligence whistleblower, released documents in 2013 showing that the US and its allies were conducting sweeping Internet and phone data surveillance in European Union countries, including in Germany.

The federal prosecution said, however, that there was no hard evidence showing that intelligence services illegally, systematically and massively monitored German telecommunications and Internet traffic.

It said the US spy service had, in fact, used techniques and capabilities to conduct strategic signals intelligence and tap online communications, but it said the so-called Snowden documents provided no concrete evidence of actual espionage activities carried out by the NSA in or against Germany.

The prosecution said the conclusion was based on a thorough assessment by the investigators in partnership with Germany’s BfV domestic security service, which handles counter-espionage, the federal IT security agency BSI; the NSA parliamentary inquiry; and the operator of a major European internet hub in Germany.

The US espionage on Germans sparked a huge controversy when revelations about the case emerged in 2013. Chancellor Angela Merkel was even forced to warn the White House that spying between allies was unacceptable and Berlin could take actions.

The warning came after local media said the NSA had tapped Merkel’s mobile phone.

The scandal further deepened in 2015 when it became clear that Germany’s intelligence service, BND, had helped the NSA eavesdrop on senior EU officials.