German police have arrested a suspected far-right extremist who they say sent dozens of death threats to German politicians signed with the name “NSU 2.0” — a reference to a notorious neo-Nazi group from the 1990s.

The case has made huge waves in Germany and rung alarm bells about the growing strength of the far right. It comes at a time when Germany is seeing a surge in extremist attacks on elected politicians and public servants.

According to official statistics released on Tuesday, Germany saw 23,604 politically-motivated offences with a rightwing background last year, the highest number since records began in 2001.

For nearly three years, police have been trying to identify the author of a series of threatening letters targeting MPs, media figures and lawyers — many of them women with an immigrant background.

They were sent under the pseudonym “NSU 2.0” — a reference to the National Socialist Underground, a neo-Nazi cell whose bloody campaign of killings, bombings and arson attacks claimed the lives of 10 people between 2000 and 2007.

The first known NSU 2.0-related threats were received in August 2018 by Seda Baysal-Yildiz, a German lawyer of Turkish descent who represented one of the victims of the original NSU group. The author threatened to kill her and her then two-year-old daughter.

Investigators later discovered her name had been searched for in a police database in the western state of Hesse, suggesting collusion with law enforcement agencies. Janine Wissler, leader of Die Linke, a leftwing party, and the comedienne Idil Baydar were also targeted.

A statement from prosecutors in Frankfurt and the police authority in the western state of Hesse on Monday said investigators had searched a flat in Berlin and taken a 53-year-old unemployed man into custody.

It said the man was suspected of having sent a series of letters with content of a “threatening, insulting nature, [designed to] incite racial hatred”. He had previously been convicted of other offences of an extremist nature.

Police and prosecutors are examining the man’s computers and continuing to investigate him for suspected incitement of racial hatred, using the symbols of a banned organisation, threats and verbal abuse.

Authorities in Hesse said in March that a total of 133 threatening letters had been sent, 115 of which could be attributed to the NSU 2.0 case and 18 to copycats. The 115 messages were sent to 32 people and 60 institutions in nine of Germany’s 16 states, as well as Austria. Most of them were emails, but some were sent by fax, SMS or via internet contact forms.

It remains unclear how the suspect acquired the personal data, including home addresses, of the people he targeted. A person close to the investigation said it was assumed he posed as a policeman in order to procure the information from local registry offices.

The statement stressed that the arrestee had never been employed by a police authority, either in Hesse or elsewhere in Germany.

Peter Beuth, Hesse’s interior minister, said Monday’s arrest exonerated the Hesse police. “Judging by everything we know today, no Hesse policeman was ever responsible for the NSU 2.0 threatening letters,” he said.

The investigation was complicated by the many copycats who employed the NSU 2.0 username to exploit the huge public interest aroused by the case. In the summer of 2020, a former policeman from the southern state of Bavaria was arrested on suspicion of sending threatening letters, some of which were directed at Bundestag MPs.