BERLIN (AP) — A German man was sentenced Thursday to nearly six years in prison for sending threatening letters to politicians, lawyers and journalists signed with the acronym of a neo-Nazi group.

A court in Frankfurt found the 54-year-old man guilty on a range of charges including inciting crime, inciting hatred, disturbing the peace, issuing threats, and assaulting a law enforcement officer, according to the news agency dpa.

The man, who was identified only as Alexander M. in keeping with German privacy rules, received five years and 10 months in prison, dpa reported.

Starting in 2018, Alexander M. sent dozens of letters via email, fax and SMS message to individuals across Germany and Austria. The letters were signed “NSU 2.0,” using an acronym for a neo-Nazi group called the National Socialist Underground.

The NSU 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 police officer. Its name was derived from the full name of the Nazi, or National Socialist, party.

Among those who received the letters were Frankfurt-based human rights lawyer Seda Basay-Yildiz, entertainer Jan Boehmermann, television moderator Maybrit Illner and cabaret artist Idil Baydar.

Alexander M. denied sending the letters at the trial, dpa reported. He said they were on his computer because of his membership in a chat group on the dark web.

The case comes as German security agencies have been warning of the growing threat of violent far-right extremism. Germany’s domestic intelligence service said this summer that the number of people active in the right-wing extremist scene in Germany has risen to 33,900.

A series of high-profile attacks by right-wing extremists have also drawn attention to the issue in recent years.

In July 2019, a regional politician from former 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. And in early 2020, a right-wing extremist killed nine people of immigrant backgrounds at two hookah bars in central Germany.