BERLIN — Germany’s most notorious far-right politician, Björn Höcke, a man who has called the Holocaust memorial in Berlin “a monument of shame,” was furious when he woke up one morning to find a section of the memorial built to scale outside his bedroom window.

It was November 2017. Mr. Höcke sued the artist who had secretly rented the neighboring property and prepared the stunt over many months. He tried to have him evicted. He even launched criminal proceedings against him. But to no avail. The rental lease is open-ended.

And the replica of the Holocaust memorial remains.

But where civil and criminal proceedings have failed, a state prosecutor has stepped in, it emerged Wednesday. The prosecutor’s office in Mr. Höcke’s eastern home state, Thuringia, is investigating the artist, Philipp Ruch, on suspicion of “forming a criminal association,” a legal provision known as Paragraph 129, which gives the state far-reaching surveillance rights and is normally applied to criminal groups and suspected extremists.

The investigation, which was revealed in the answer to a routine parliamentary question, is now raising questions about whether the law is being leveraged by far-right sympathizers to crack down on their opponents. Mr. Höcke’s party, the far-right Alternative for Germany, the country’s third-largest party in Parliament, is particularly strong in the former Communist East of the country.