BERLIN — Police in Berlin have arrested a 53-year-old German man on suspicion of sending dozens of threatening letters to politicians, lawyers and journalists that were signed with the acronym of a far-right extremist group.

Prosecutors in Frankfurt, who are handling the case, said early Tuesday that the man was detained in the German capital on Monday.

The suspect, whose name wasn’t released for privacy reasons, has previous convictions for “numerous crimes, including ones that were motivated by right-wing ideology,” prosecutors said.

The letters were signed “NSU 2.0” – a reference to the National Socialist Underground group that was responsible for a string of violent crimes between 1998 and 2011, including the racially motivated killings of nine men with immigrant backgrounds and a policewoman.

Police think the suspect sent almost 100 letters to dozens of people and organizations across Germany and Austria since 2018.

German security agencies warned of the growing threat of violent far-right extremism. In July 2019, a regional politician from Chancellor Angela Merkel’s party was killed by a neo-Nazi; three months later, a gunman tried to force his way into a synagogue on Yom Kippur, killing two people.

In February of last year nine people with immigrant backgrounds were shot dead in Hanau, near Frankfurt, by a gunman who had called for genocide.