"If reports are correct, we are not talking here about small potatoes," Foreign Minister Frank-Walter Steinmeier said in a tweet, following reports of US spying that have sparked anger in Germany after revelations the NSA allegedly tapped Chancellor Angela Merkel's phone.
The US ambassador, who was called to a meeting at the foreign ministry late on Friday, had been told Washington is expected to shed light on the reports "as quickly as possible," he added.
The 31-year-old employee of the German foreign intelligence agency known as the BND arrested last week had been working for the CIA for around two years, local media reported today.
"All signs indicate that he was acting for the Americans," the Frankfurter Allgemeine Sonntagszeitung (FAS) Sunday newspaper quoted an unnamed senior official at Germany's foreign intelligence service as saying.
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Both newspapers said the suspect had passed on two documents about a parliamentary panel established earlier this year to investigate NSA surveillance after revelations by fugitive former intelligence contractor Edward Snowden.
German authorities were alerted to the suspect when he sent an email with attached files at the end of May to the Russian consulate offering to supply information, reports said.
President Joachim Gauck said in excepts from an ZDF public TV also released early that, should the suspicions about the US be confirmed, "then it probably really has to be said, now it's enough".