Unexpected inquiry, announced by federal prosecutor, will determine if US actively listened in to calls

Germany's federal prosecutor has defied public expectations by opening an investigation into the alleged tapping of Angela Merkel's mobile phone by the US's National Security Agency (NSA).

Federal prosecutor Harald Range announced on Wednesday: "I informed parliament's legal affairs committee that I have started a preliminary investigation over tapping of a mobile phone of the chancellor."

Merkel had complained to Barack Obama in person about the alleged tapping of her phone last October, but the federal court's investigation, which will be against unnamed persons, would constitute the first formal response to the affair. The German government has reportedly announced its support for the investigation.

The Karlsruhe-based court's decision comes as a surprise, not least since it appeared that both the German and the US governments had over recent months successfully calmed the waves stirred up by the revelations.

During Merkel's visit to Washington in May, the NSA affair had been largely sidelined by the Ukrainian crisis, and an attempt to invite Edward Snowden as a witness to the Bundestag's own inquiry into NSA surveillance appeared to have been successfully blocked by members of Merkel's own government, with the justification that an invitation would have put a "grave strain" on US-German relations.

Only a few days ago, several media outlets had reported that the federal prosecutor's investigation too would be dropped. The considerable public outrage about the information leaked by NSA whistleblower Edward Snowden – evoking memories of the Stasi's state surveillance among many German citizens – appeared to have ebbed away.

The key issue for the prosecution will be to establish whether the NSA monitored the German chancellor's mobile automatically or by default, as the US government has so far implied, or whether individual agents were actively engaged in tapping her calls, as German tabloid Bild claims on Thursday.

The latter would constitute a clear breach of German law on German soil according to paragraph section 99 of the German criminal code.

The federal prosecution has stated that it has currently no plans to look into the alleged wider surveillance of German citizens through US intelligence services, a decision which some politicians have been quick to criticise.

Social Democratic party politician Ralf Stegner told Handelsblatt newspaper: "Orwell's 'All animals are equal but some animals are more equal than others' can't be the right motto when dealing with a massive and mass-scale breach of civil rights".

However, Range stated that the scope of the investigation could be widened if the federal prosecutor were to obtain new evidence relating to general surveillance.

Konstantin von Notz, a Green party MP and member of the Bundestag's NSA inquiry committee, welcomed the federal court's move but suggested that it should eventually be broadened out to include surveillance of normal citizens.

"I assume that the tapping of Angela Merkel's cell phone must have involved active decisions by real people, whereas the monitoring of 80 million Germans would have been a matter of algorithms and people analysing the results. Both actions amount to violations of German law. Whether you have a human being or a computer opening and scanning your letters for individual words, it's the same thing, because government authorities have gained this information," von Notz said.