Leipzig's mayor, Burkhard Jung, freshly elected president of the Association of German Cities, on Friday referred to the June 2 death of Walter Lübcke at his home near Kassel as an "execution."

Lübcke, who was a member of Chancellor Angela Merkel's center-right Christian Democrats and headed the administrative district of Kassel in the state of Hesse, died on June 2 after a close-range nighttime gunshot to his head.

"The murder or indeed the execution of Walter Lübcke must be cause for nationwide discussion," Jung told the Frankfurter Allgemeine Zeitung (FAZ) newspaper.

Lübcke was found severely wounded on his house terrace and died in the hospital shortly afterwards.

In 2015, during Europe's crisis over refugee arrivals, Lübcke had publicly called for a welcoming stance toward asylum-seekers and was subjected to a backlash of far-right messages on the internet.

Last Saturday, a police squad arrested a 45-year-old suspect, named only as Stephan E. under Germany's reporting laws, at his home in Kassel. The suspect had convictions from past decades, including one for an assault on a hostel for asylum-seekers in 1993.    

Federal prosecutors, who this week took over the case, have rated Lübcke's death as a political slaying with a right-wing extremist background.

Mayors facing surge in slander, violence

Jung, a Social Democrat (SPD) and the first eastern German mayor to be elected president of the Cities Association, told FAZ on Friday that especially local politicians had been exposed to an "unbelievable surge in insults and slander and even murder threats and violence" since 2015.

"Every day, about three politically motivated crimes are committed against political officeholders, in particular against local community leaders," Jung told German news agency dpa.

Many local politicians no longer dared to go public on issues, fearing their utterances would worsen the situation. Similarly abused were firefighters, ambulance crews and local authority employees, said Jung. "It's almost unbearable."

Read more: German mayor quit in 2015 after anti-asylum campaign

"We have a deep split in society," Cities Association chief executive Gerd Landsberg told Oldenburg's Nordwest-Zeitung newspaper, adding that it remained even though Europe's migration crisis of 2015 no longer topped Germany's political agenda.

Hamburg Parliament president also threatened

Among the latest officeholders to disclose threats was the president of the Hamburg Parliament (Hamburgische Bürgerschaft), Carola Veit of the center-left Social Democrats (SPD).

"I personally have also received murder threats in the last few days ... That is disgusting and obnoxious," Veit told Radio Hamburg, a private station, adding that it was outrageous that her children had also been threatened.

A call to reopen investigators' files and to examine whether the Lübcke case might be linked to another Kassel case — the murder of Halit Yozgat in 2006 — has come from Barbara John, German government commissioner for victims of a neo-Nazi ring that called itself National Socialist Underground, or NSU. It went underground in 1998 when police found crude pipe bombs in a garage in Jena.

The NSU went on to murder nine migrant residents — most of Turkish origin — between 2000 and 2006 in six cities across Germany. The shooting of a policewoman in Heilbronn in 2007 was also attributed to the NSU killer cell.

After a long Munich trial, Beate Zschäpe was convicted of crimes including murder and arson. Two other main suspects had been found dead in 2011 in a burned-out camper in Eisenach, complete with a pistol traced to the killings.

Referring to the murder of Yozgat, John told Germany's RND network: "I don't rule out that the man now under arrest [Stephen E.] might have then belonged to NSU circles behind it."

"That's why the murder case files in Kassel [in Yozgat's case] need to be reopened," said John, referring to the insistence by the state of Hesse's intelligence service on keeping the records secret — ostensibly to protect informants in NSU circles.

A 'declaration of war,' posits NSU investigator

Specialist Berlin-based journalist on political extremism Andreas Förster, writing in the leftist cultural weekly der Freitag and co-author of a compendium on the NSU trial, said Wednesday German authorities must not repeat "mistakes" made in the NSU investigation when probing what he termed the Lübcke murder.

"After the murder of Walter Lübcke, the state must finally understand the interplay of network structures and autonomous cells in the right-wing scene," said the introduction to Förster's article.

"If neo-Nazis switch to the targeted killing of politicians, this should be understood for what it is and taken seriously: a declaration of war on the state and democratic society," Förster concluded.

Further terror acts likely

Criminologist Tobias Singelnstein of Ruhr University, Bochum, told dpa on Thursday it was reasonable to assume that five years of mobilization by the extreme right in Germany would result in further acts of terrorism.

"I regard that as only a matter of time," Singelnstein said.