The gunman who killed two female police officers and a bystander in the Belgian city of Liege on Tuesday had killed a man the previous day, Belgian Interior Minister Jan Jambon told broadcaster RTL.

Jambon confirmed that the victim was a former inmate who did prison time with the assailant, identified as a Belgian national called Benjamin Herman. The attacker is alleged to have killed the man by hitting him over the head with a blunt object.

Watch video 00:34 Gunman kills two policemen in Belgium

During Tuesday's attack, Herman, who was on a two-day release from prison, approached the two female officers from behind and stabbed them several times, before seizing one of their weapons and shooting them dead.

He then shot dead a 22-year-old man in a vehicle parked nearby and fled to a nearby school where he took a woman hostage.

Herman was later killed in a shoot-out with police.

An investigation has been launched into the incident, including the circumstances surrounding the attacker's release from prison, Jambon said.

"It's really an isolated case. He wasn't part of a network, he did not receive instructions from anyone else, so there is no need to raise the terror threat alert level," Jambon said.

Act of terror

Later in the day, a Belgian federal magistrate said investigators were treating Tuesday's attacks as a "terrorist murder" and were trying to ascertain whether the attacker acted alone.

The magistrate, Wenke Roggen, confirmed reports that the attacker had shouted "Allahu akbar," the Arabic phrase for "God is great," several times during the shooting rampage. She said the attacker had been radicalized in prison.

Roggen said Herman's modus operandi resembled that of the "Islamic State" militant group, which exhorts its supporters via video messages to attack police with knives and steal their weapons.

ap/rt (AP, dpa)

Every evening at 1830 UTC, DW's editors send out a selection of the day's hard news and quality feature journalism. You can sign up to receive it directly here.