The email to Holger Kelch couldn’t have been clearer. “It said there are plenty of trees around town, enough to hang each of your family members from,” the mayor of Cottbus recalls.

This chilling threat came a year ago, after Mr Kelch appealed for calm in the north-eastern town in the wake of a Syrian refugee's arrest over the murder of an elderly woman.

Death threats against local politicians have become commonplace in Germany since Berlin decided to take in hundreds of thousands of refugees in 2015.

Local officials have learned to cope with abuse as part of their job. But this week's arrest of an alleged far-Right extremist for the murder of a regional politician has put the threats into sharp focus.

Walter Lübcke, head of government in the central German town of Kassel, was shot in his garden on June 2. He had become a regular target of far-Right abuse after he was filmed in 2015 telling critics of Berlin’s refugee policy that they were free to leave the country.

Initially police said they didn't have a motive, leaving open the possibility that Lübcke had been killed in a private feud.

But the investigation turned decisively when detectives found the DNA of a criminal with a long neo-Nazi history at the murder scene. The 45-year-old man arrested has allegedly blamed his action on his anger at an influx of refugees and migrants to Germany.