The gunman who killed at least three people in Belgium was a convicted criminal believed to have been radicalised behind bars.

Benjamin Herman had been released from jail on temporary leave just hours before the attack.

He killed two policewomen and a man passing by in the city of Liege before being shot dead by police.

Authorities suspect the 36-year-old also murdered a former associate found dead at his home.

Herman had been in and out of jail for a variety of crimes since 2003.

Belgian authorities faced questions on Wednesday over why the prison inmate was let out for a day.

The justice minister, who oversees the prison service, said he felt "responsible" for Tuesday's bloodshed in which two policewomen and a man were killed. The attacker was later shot dead by police.

"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," Koen Geens told RTBF radio. "I have to examine my own conscience."

A Belgian security source told Reuters he converted to Islam while in detention and was under suspicion of radicalisation. Terrorism is being investigated as just one possible motive for Tuesday's attack.

Herman stabbed the two officers from behind with a knife, before seizing one of their weapons and shooting them.

He went on to shoot a 22-year-old man sitting in a car before entering a nearby high school and taking two women hostage.

As he burst from the building, he shouted 'Allahu Akbar', while shooting at police.

Federal investigators have taken over the case - an indication they suspect terrorism as a motive.

Belgium has been on high alert since a Brussels based Islamic State cell carried out attacks in Paris in 2015 and in Brussels the following year.