The three-minute video, posted on an Islamist page, praises the "lion Murat K." and promises, "We will not rest until we have freed you from captivity."
"Everyone who offends the Prophet will be slaughtered, whether near or far," a man says in German with a foreign accent. "And know, brother, the Germans are near enough to reach. We will take them prisoners, until you are free for your noble deed."
Der Spiegel magazine reported on Thursday that the man speaking the voice is former Berlin rapper Denis Cuspert alias Deso Dogg. He is thought to have fled to Egypt earlier this year to re-establish the militant organization Millatu Ibrahim, which is banned in Germany, and whose logo appears in the video.
German security forces are taking the video seriously and investigating its source. "Such videos could spur on Salafists prepared for violence," said North Rhine-Westphalian Interior Minister Ralf Jäger. "That is why the authorities are keeping the protagonists of this scene in their sights."
A spokesman for the federal state prosecutors said they were aware of the video and analyzing it.
A Bonn court convicted Murat K. of grievous bodily harm, breaching the peace, and resisting arrest in October. He stabbed the two policemen when a demonstration turned violent after members of the far-right group Pro-NRW publicly displayed cartoon depictions of the Prophet Muhammad.
Murat K. defended his actions in court, saying that, "Those who insult the Prophet deserve the death penalty." He added that since the state had allowed the cartoons to be displayed, he was justified in attacking the officers.
Federal prosecutors said last week they believed Islamist extremists were behind a botched bomb attack at Bonn train station. The federal prosecutor said in a statement that there was enough evidence to suggest the incident was "an attempted explosives attack by a terrorist organisation with a radical Islamist bent."