The Austrian prosecutor's office is investigating an Islamic extremist on suspicion of murder after a video was posted showing him shooting a kneeling man to death in Syria.

The video surfaced last week. Prosecutors' spokeswoman Nina Bussek said Friday her office considers it genuine.

Mohamed Mahmoud, the suspect, is already being sought by Austrian authorities on an international warrant on charges of belonging to a terrorist organization, and Bussek says her office has asked a court to add suspicion of murder to the warrant.

Mahmoud is considered one of the most important German-speaking advocates of Islamic extremism. He was born in Vienna but left Austria in 2013, first for Germany where he was part of the radical scene and then for Syria, via Turkey.