A Belgian prison inmate who killed four people while on a two-day furlough committed "terrorist murder" and likely intended to cause more harm, prosecutors said Wednesday as authorities searched for possible accomplices and the Islamic State group claimed responsibility for the bloodshed.

The convict who stabbed police officers in the city of Liege and used their handguns to kill them and a bystander was a "soldier of the caliphate," ISIS said in a brief statement on the site of its Aamaq news agency, the Associated Press reports.

Benjamin Herman, the Belgian national named as the "Liege killer," was a convert to Islam. He had been imprisoned since 2003 and was on a 48-hour leave when the two female police officers were attacked. Police shot him dead not long after.

Herman first stabbed the officers repeatedly from behind with a knife, stole their handguns and shot them as they lay on the ground. Crossing the road, he shot a 22-year old passenger in a car and shouted "Allahu akbar," the Arabic phrase for God is great, several times, authorities said.