A suspected German double agent has worked for the U.S. Central Intelligence Agency (CIA), German newspaper “Bild am Sonntag” reported on Sunday.

The case was first uncovered on Friday. A 31-year-old employee of Germany’s Foreign Intelligence Service (BND) was reportedly detained Thursday in suspicion of having spied on a German investigation committee inquiring into U.S. surveillance on behalf of an American intelligence service.

The German stated he had been sending secret documents once a week and passed on a total of 218 documents to the United States, “Bild am Sonntag” reported.

The report said the BND employee, whose last task was to get information from the German parliament’s panel for investigation of spying activities of the U.S. National Security Agency (NSA), was apparently “accurately controlled” by U.S. authorities.

According to German newspaper “Frankfurter Allgemeine am Sonntag”, the man offered himself per e-mail to the U.S. Embassy in Berlin and was being led by a U.S. intelligence service as agent since the end of 2012.

The case has sparked mass indignation among German politicians, who are demanding explanation and warned of negative consequences for the transatlantic relationship.

German President Joachim Gauck expressed his outrage over the case in an interview on Saturday, saying it is “a game with friendships and close relationship” should the suspicion is confirmed.

German government spokesman Steffen Seibert said Friday that the government was concerned about the case, describing it as “very serious.” Chancellor Angela Merkel was informed about it on Thursday, Seibert added.

Investigators are currently reviewing the suspect’s words. If the suspicion is confirmed, this case would be the biggest scandal involving a German-American double agent in the postwar period.

Revelations of U.S. data gathering practices, especially allegations about its tapping of Merkel’s mobile phone, have led to explicit criticism in Germany and strained relations between Germany and the United States.

The NSA inquiry panel was set up by German parliament in March to take a close look at the U.S. surveillance activities as well as the role the BND has played in the spying scandal.