When the command-structure for the Sicherheitspolizei (SiPo) in Norway was altered in February 1945, Fehmer was made head of Division IV (the Gestapo) in the new Oslo branch.

[1][2] Following the end of the war in Europe on 8 May 1945, Fehmer disguised himself as a Wehrmacht soldier and mixed in with other German POWs at Haslemoen Camp in Våler.

He was subsequently betrayed and arrested after making a telephone call to a female friend to inquire how his pet German Shepherd was doing.

[2] During his trial for war crimes, Fehmer attempted to avoid execution by proving that he had managed to help a handful of prisoners and by presenting himself as a valuable source of information.

[1][2] Fehmer was the main antagonist in the 2008 Norwegian war film Max Manus, where he was portrayed by German actor Ken Duken.