Two police officers were killed on Tuesday when an assailant opened fire at them in the center of the Belgian city of Liege.
The perpetrator also shot a passer-by before being shot and killed by police.
The shooting happened near a cafe on the Boulevard d'Avroy, public broadcaster RTBF said on its website.
The gunman fled the scene, taking a cleaning woman hostage at a nearby school, but he was later "neutralized," RTBF reported, quoting police and fire services.
Two other police officers had been injured, Belga news agency said.
The national anti-terrorist crisis center, which Interior Minister Jan Jambon said on Twitter was monitoring the situation, said terrorism could not be excluded as a motive though it was also looking into other possible reasons.
"It (terrorism) is one of the questions on the table, but for the moment all scenarios are open," a spokesman for the crisis center said.
Speaking on Belgian television, Prime Minister Charles Michel said: "There was a serious incident. The information so far is not clear yet."
He was rushing to the crisis center to get more information.
"We don't know anything yet," Catherine Collignon, spokeswoman for the Liege prosecutors office, told AFP when asked about the shooter's motives.
Belgium terror prosecutors later took over the case.
Liege, an industrial city close to the German border in a French-speaking region, was the scene of a shooting in 2011, when a gunman killed four people and wounded more than 100 others before turning the gun on himself.
Belgium has been on high alert since a Brussels-based ISIS cell was involved in attacks on Paris in 2015 that killed 130 people and Brussels in 2016 in which 32 died.
