Belgian authorities face questions over why a prison inmate, suspected of links to radical Islamists in jail, was let out for a day, enabling him to carry out what police called a "terrorist attack" in the city of Liege.

A gunman kills two police officers and a passer-by on Tuesday before being shot dead in the centre of the Belgian city of Liege, public broadcaster Radio Television Belge Francophone (RTBF) says on its website and authorities says terrorism could not be ruled out as a motive.