Belgian investigators said on Wednesday the ISIS-style attack in the city of Liege on Tuesday was an act of terrorism. Benjamin Herman, an inmate on a two-day release, attacked two female police officers with a knife from behind, stabbing them repeatedly, before stealing their weapons and shooting them as they lay on the ground, a method investigators said was encouraged in online videos by the ISIS terrorist group. Crossing the road, he fired several shots at a 22-year old man who was a passenger in a car, killing him. Herman then took at least one woman hostage at a nearby school. When police closed in, he ran out onto the sidewalk firing and police fatally shot him. Police were scrambling to unpick his motives. Herman was a 31-year-old drifter with a decade spent in and out of prison for acts of violence and petty crimes. Authorities were also determining whether he worked alone. "The facts are qualified as terrorist murder and attempted terrorist murder," prosecutors spokesman Eric Van Der Sypt told a news briefing in Brussels. Van Der Sypt said the assessment was based on several "first elements" from the probe, such as information from state security according to which the perpetrator was in touch with radicalized persons. But he cautioned that the information dated "from late 2016, early 2017" and had not been confirmed since. Prosecutors also underlined that the attackers method -- attacking armed police officers and using their weapon against them -- was a known "modus operandi" of ISIS, which claimed deadly attacks in Brussels in 2016. But Interior Minister Jan Jambon urged caution over the extremist angle. "There are signals that there was radicalization in the prison but did this radicalization lead to these actions? There too we can ask ourselves a lot of questions," he told RTL radio. Special attention is being given to the gruesome killing of an alleged heroin dealer linked to Herman who was bludgeoned to death with a hammer late Monday in a village near the Luxembourg border. Investigators on Tuesday found the hammer in Hermans car and Jambon said police believed the Liege attacker carried out the killing just hours after getting temporary release from prison. Prosecutors confirmed Herman was being investigated over the case, saying it was a separate investigation. The two murdered police officers were identified as Lucile Garcia, 53, who had recently become a grandmother, and Soraya Belkacemi, 45, a mother to 13-year-old twins. Debate in Belgium was swirling on the countrys prison policy with reports that Herman had repeatedly blown the conditions of his temporary leave from jail ahead of his full release set for 2020. "I feel responsible because I have responsibility for prisons," Belgian Justice Minister Koen Geens told RTBF radio. Liege, a major city in Belgiums blighted industrial rustbelt, was the scene of another bloody shootout in 2011 when a recent convict killed six people and wounded more than 120 before turning the gun on himself. Liege police on Tuesday said it was "clear that the assassins objective was to attack the police" and that one of the four officers wounded had suffered a serious leg injury. Prime Minister Charles Michel denounced what he called the "cowardly and blind violence" of Tuesdays attack. Belgium has been on alert since authorities in January 2015 smashed a terror cell in the town of Verviers near Liege that was planning an attack on police. The cell also had links to Abdelhamid Abaaoud, the mastermind of the November 2015 ISIS attacks on Paris that killed 130 people. ISIS suicide attacks then targeted Brussels airport and a metro station, leaving 32 people dead in March 2016.