The man who shot dead two police officers and a civilian in the Belgian city of Liège had killed someone the night before the attacks, the country's interior minister says.
Jan Jambon said the gunman, named as Benjamin Herman, had murdered a man - a former prisoner - he met while in jail.
Reports said the victim was Michael Wilmet, 30, a convicted drug dealer who was hit repeatedly with a hammer.
Prosecutors say they are treating the attacks as "terrorist murder".
The gunman also took a female cleaner hostage at a school before being killed by police.
Officials said the 31-year-old man had shouted "Allahu Akbar" ("God is greatest" in Arabic) several times during the attack.
He was let out from prison on temporary release on Monday where he had been serving time for theft and drug offences. Local media report that he may have been radicalised while in jail.
"There are signs he was radicalised in prison, but is it that radicalisation which drove him to commit these acts?" Mr Jambon said on RTL radio.
"It could have been because he had nothing to look forward to, because he also killed someone the night before, the guy's psychology and the fact... he may have been on drugs."
Why was he released?
The gunman had been in and out of prison since he was a teenager and had a criminal record that included assault and drug offences, police say.
He was released at 07:30 local time on Monday and, under the terms of the temporary release, was expected to return to jail on Tuesday. It was during this period that he launched the attacks.
Justice Minister Koen Geens defended the decision to release him, saying it was intended to help him prepare for his full release scheduled for 2020. He added that the attacker had been granted temporary leave numerous times before without incident.
But he admitted that he had to "examine" his conscience following the attacks.
"The question of whether this man should have been given leave is striking because he killed three completely innocent people with a wish to kill himself," he told RTBF radio.
"I feel responsible because I have responsibility for prisons."
What happened?
The shooting unfolded late on Tuesday morning near a cafe in the city centre.
Prosecutors said the man followed and "savagely" attacked two female police officers, aged 44 and 54. He stabbed them repeatedly from behind with a knife before taking a gun from them and opening fire.
He also shot dead a 22-year-old man who was sitting in the passenger seat of a parked car. He then walked to a nearby school, where he briefly took a member of staff hostage.
When armed police officers arrived at the scene, the attacker fired at them in an attempt to escape. Two officers were shot in the leg, leaving one officer in a serious condition.
Two other officers received injuries to their arms before the attacker was shot dead, prosecutors said.
Footage on social media showed people running to safety as several gunshots rang out. Children at the school were moved to safety.
Terrorist alert
Belgium remains on alert after a series of jihadist attacks in the country and in neighbouring France.
A Brussels-based cell was involved in the 2015 Paris attacks which left 130 people dead in several locations.
Brussels itself saw three suicide bombings in March 2016, with 32 people killed. Both the Paris and Brussels attacks were claimed by the Islamic State group.
Later in 2016 a man attacked two police officers with machetes while shouting "Allahu Akbar" before being shot dead.