A new surveillance scandal is threatening to unsettle US-German relations after it emerged that an employee of Germany's intelligence agency has been arrested under suspicion of acting as a double agent for the US.

According to several reports in the German media, a 31-year-old member of the Bundesnachrichtendienst (BND) administration department in Pullach was on Wednesday arrested by the country's federal prosecutor, originally under suspicion of passing on information to Russian intelligence services.

However, under questioning by the federal prosecutor the suspect said he had received money in exchange for passing on secret information to a US contact. If his claims turn out to be true, German papers say it would constitute the biggest scandal involving a US-German double agent in the post-war era.

Some newspapers are speculating whether the BND employee may have been specifically tasked with spying on the activities of the special Bundestag inquiry committee currently investigating the NSA's activities in Germany.

According to Süddeutsche Zeitung newspaper, the employee had been approached several times by the NSA, at least once with a specific request for information on the Bundestag's investigation into NSA surveillance.According to Der Spiegel, the BND staffer had collected between 200 and 300 secret documents from internal servers and saved them onto a USB stick.

They were sold on to the US intelligence services between 2012 and 2014, for price of several tens of thousands of euros, said the magazine. The employee had managed to establish contact with the NSA by the most obvious way imaginable – by sending an email to the US embassy.On Friday, the investigative committee gathered for an emergency meeting in response to the arrest. Martina Renner, a Left party politician on the parliamentary committee, told Associated Press that the case indicated that anyone who examined Snowden's revelations in detail was subject to scrutiny by US intelligence agencies.

A spokesperson for Angela Merkel said that the chancellor had been informed of the arrest, and that allegations of espionage "weren't something that was taken lightly".

News of the double-spying allegations come less than a day after two former NSA employees spoke as witnesses to the Bundestag inquiry. William Binney, a former technical head, said the NSA had a "totalitarian mentality", claiming that it represented the "greatest threat" to American society since the civil war.

Thomas Drake, another former NSA staffer who worked at the agency between 2001 and 2008, accused Germany's BND of collaborating close with their American counterparts. The German intelligence agency's claims that they had been unaware of NSA surveillance activity in Germany were, he said, "beyond credibility".