The German government is considering countersurveillance measures against British and American intelligence officials in Germany for the first time since World War II in the wake of news that a German spy turned against his own government, according to numerous sources.

Chancellor Angela Merkel’s administration is weighing a variety of options, including increased intelligence-gathering on NATO allies such as the U.S., the United Kingdom and France, as well as forcing suspected American agents out of the country, according to the Guardian. Thomas de Maiziere, the minister of the interior, told the German-language publication Bild that Germany needs a “360-degree vision” of foreign spying, with Bild also claiming it has obtained a document that describes possible “concrete countermeasures” against NATO allies.

While German lawmakers have been outraged by the spying on Merkel and others exposed by former U.S. National Security Agency contractor Edward Snowden since last summer, their frustration has grown since the federal prosecutor arrested a 31-year-old employee of the German spy agency, the BND, who allegedly sold secrets to the CIA.

Reuters reported on Monday that the CIA was involved in recruiting the unnamed suspect, who admitted to sharing details about a German parliamentary investigation into NSA surveillance. It’s been reported that he made initial contact with the American Secret Service through the U.S. Embassy in Berlin, but instead of reporting the breach to German officials, the CIA paid the source 25,000 euros (nearly $34,000) for 218 top-secret documents. The man was “not a top agent,” according to multiple German politicians.

“If the reports are correct, it would be a serious case,” Merkel told a press conference while traveling in China. “It would be a clear contradiction of what I consider to be trusting cooperation.”

A spokesperson for the Interior Ministry didn’t deny that Germany is trying to assess the best way to proceed, telling the Guardian that the country recognizes “an efficient and effective counterintelligence against all sides is important, necessary and has to be better organized than it has [been] until now.”

At immediate risk is Germany’s cooperation in the so-called gentlemen’s agreement among Germany, Britain, France and the United States that they would not spy on each other, an unwritten promise that originated at the end of World War II. Instead of collecting information on each other, the rule said, each nation should share the intelligence it gathered first on the Soviets and then, after the Cold War, various other nations.

“When a friendly intelligence service breaks these rules, it commits the most serious breach of trust known to the world of espionage,” Munich’s Süddeutsche Zeitung newspaper declared in a recent editorial.

Speculation has centered on the idea that Germany will at least focus more of its surveillance power on “listening posts” on the roofs of the U.S. and British embassies in Berlin, located not far from the Reichstag parliament building.

White House press secretary Josh Earnest offered no comment, saying only that the U.S. is committed to continued collaboration.

“The relationship that the United States has with Germany is incredibly important. This is a very close partnership that we have on a range of security issues, including some intelligence issues,” he said earlier this week. “All of these things are high priorities not just to this administration, but to this country. So we’re going to work with Germans to resolve this situation appropriately.”

The notion that the best way for the Germans to respond is to conduct their own espionage operations has been a point of contention, though, with some lawmakers arguing that a different course might be a more effective deterrent.

“To respond to these allegations with the motto ‘Now we’re going to spy back on you’ is just absurd, and a sign of the government’s helplessness. Either these surveillance activities are illegal or we do something to curb them, or not,” said Konstanin von Notz, a member of the parliamentary investigative committee into the NSA activity from where the German documents were leaked, as quoted by the Guardian.

“If the German government wanted to apply real pressure on the U.S., it would do something about the [the trans-Atlantic trade agreement] TTIP or the Safe Harbor directive," a European Union measure to prevent accidental information disclosures.

The Reuters report on Monday also noted that CIA Director John Brennan, while making no public comment, had offered to brief U.S. senators on the agency’s involvement with the suspected double agent. Such a meeting would come at a time when tension between Brennan and members of the Senate Intelligence Committee is at an all-time high over the details of a classified Senate report on the CIA’s post-9/11 “enhanced interrogation” program.