Germany's domestic intelligence agency has put the country's largest opposition party under surveillance as a potential threat to the country's constitution, according to public broadcaster ARD and other media outlets. The move affects dozens of lawmakers who are in the right-wing Alternative for Germany, or AfD, party.

Members of the AfD say the investigation is politically motivated, aimed at weakening the party. In January, one party leader accused Chancellor Angela Merkel's government of "trying to stigmatize us and to really put us in the Nazi corner."

The Bundesamt für Verfassungsschutz, the Federal Office for the Protection of the Constitution, is not commenting publicly on the investigation. But the designation of the AfD as a suspected case of anti-constitutional activity escalates the agency's ongoing inquiry, allowing it to start a number of surveillance operations, from cultivating informants and tapping phone calls to reading emails.

In response to the news, AfD leaders said the intelligence agency's actions against it are themselves unconstitutional.

Alice Weidel, an AfD parliamentary leader, said the investigation is "purely political." She also implied that news of the inquiry was strategically leaked to the press with important elections looming this year.

Weidel and other leaders said they will be taking legal action to fight the surveillance. AfD members serve in the Bundestag, Germany's national parliament, as well as all 16 German state legislatures and the European Parliament.

Federal investigators have been gathering information on the Alternative for Germany party for at least two years and have generated a report that's some 800 pages long, ARD reports. Some of the findings against the AfD are believed to accuse it of violating human dignity, the protection of which is enshrined in the first article of Germany's constitution.

The Verfassungsschutz agency cited human dignity violations when it placed a far-right branch of the AfD under surveillance a year ago, saying that der Flügel ("The Wing") had "extremist intentions." The agency's leader also mentioned recent hate crimes, saying vehement anti-migrant rhetoric online was leading to violence in the real world.

The intelligence agency's actions are rooted in Germany's post-war constitution, which the U.S. helped to craft.

"The driving force behind the creation of the Verfassungsschutz agency and its surveillance powers was the American-led Allied forces," NPR's Rob Schmitz reports, "who, after World War II, helped write a new German Constitution with an eye toward preventing the return of Nazi ideology."

Unlike the United States, which is bound by First Amendment free-speech protections, Germany has outlawed symbols of the Nazi era such as the swastika and the Hitler salute. The German constitution guarantees freedom of expression, but the country's laws also place limits on far-right and other extremist speech.

Of course, U.S. officials are currently grappling with their own challenges from domestic extremism. The news from Germany comes one day after FBI Director Christopher Wray condemned the Jan. 6 attack on the U.S. Capitol as "domestic terrorism," referring to the mob of former President Donald Trump's supporters who attempted to stop Congress from certifying the results of the 2020 election.

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