BERLIN (AP) — German police are investigating a series of threats sent to officials and institutions days after the arrest of a far-right extremist suspected of killing a pro-migrant politician.

Berlin police and prosecutors said in a statement Thursday that the threats were sent Tuesday.

It's unclear if they are in any way linked to the slaying of Walter Luebcke, a long-time member of Chancellor Angela Merkel's party, who was found shot in the head at his home near the central German city of Kassel on June 2.

Police arrested 45-year-old Stephan Ernst over the killing Saturday. Ernst has a string of convictions for violent, anti-migrant crime dating back to the late 1980s.

German security officials have warned that the far-right extremists pose a serious threat.