A suspect has been arrested after two men and a woman were killed and one person seriously injured in a shooting at the Jewish Museum in central Brussels.
A second suspect is still being sought in connection with the attack.
Belgium's interior minister said anti-Semitic motives could be behind the attack.
A spokesman for the Brussels fire brigade said the shooter drove up to the museum, went inside and fired shots.
Interior Minister Joelle Milquet said at the scene: "We don't yet know if they [the victims] were tourists or staff, they haven't been identified."
Asked whether she believed it was an anti-Semitic attack, she said it was too early to say with a police and judicial inquiry just under way, but that given the target "there are strong grounds for presuming so".
The area around the museum has been closed off and the security has been beefed up in places associated with the Jewish community in Belgium, Ms Milquet said.
About half of Belgium's 42,000-strong Jewish community lives in Brussels.
Security around all Jewish institutions in the country has been raised to the highest level, and Prime Minister Elio DiRupo was meeting with police and senior officials to discuss the situation.
Foreign Minister Didier Reynders, who was in the area at the time of the shooting, said witnesses had told him the suspected shooter was carrying bags when he entered the museum.
"I was in the neighbourhood. I saw two people on the ground and with other people, we called the emergency services," he told French BFM TV.
"Witnesses told me that the suspected shooter was carrying bags when he entered the museum so it was urgent for the police to check that nothing had been left inside the museum."
Maurice Sosnowski, president of the Co-ordinating Committee of Belgian Jewish Organisations said no threats have been issued to the Jewish community.
Belgium holds a general election tomorrow, with a separatist Flemish centre-right party expected to win.