Markus R. case highlights spy game between Washington, Moscow and Berlin

For this year's Independence Day bash the US embassy in Germany picked the historic Tempelhof airport, where an allied airlift 66 years ago kept Berlin's citizens from starving during Soviet leader Josef Stalin's blockade.

After a year scarred by the revelations of mass US spying on Germans, the Americans had hoped to toast US-German friendship with rock music, ribs, burgers and beer.

But that morning news had broken that Germany's federal prosecutor had arrested Markus R., a 31-year old employee of Germany's foreign intelligence agency, the BND, on suspicion of spying for the Americans.

Hours later, the German Foreign Office announced it had called in US Ambassador John Emerson to deliver "a swift explanation". For Emerson, hosting 2,500 guests that night, it was a pretty awkward party.

Revelations by former US National Security Agency contractor Edward Snowden last year that Washington spied on German officials - including bugging the phone of Chancellor Angela Merkel - had already brought relations between the US and one of its closest allies to a new low.

According to both US and German officials, Markus R. was a "walk-in agent" - someone who presents himself on his own to a foreign spy service and dangles an offer of secrets.