A gunman has shot dead two men and a woman at the Jewish Museum in the Belgian capital Brussels, BBC reported.
A fourth person was seriously wounded, emergency services said.
The attacker arrived by car, got out, fired on people at the museum entrance, and returned to the vehicle which then sped away, Belgian media report.
One person has been arrested and police are hunting a second, officials say. Security has been tightened at Jewish sites across Belgium.
Prime Minister Elio Di Rupo said all Belgians were "united and show solidarity in the face of this odious attack on a Jewish cultural site".
Belgian public prosecutor's spokeswoman Ine Van Wymersch told a news conference that one suspect had been detained at the wheel of his car, although there was no proven connection to the attack.
A second suspect was still being sought, who was thought to have fled on foot, she added.
Foreign Minister Didier Reynders, who was one of the first people to arrive at the scene, said he arrived at the scene shortly after the shooting.
"I heard bursts of gunfire, rushed here and saw the bodies on the ground," he said.
The gunman arrived at the museum at around 15:50 (13:50 GMT) carrying a backpack and opened fire before fleeing in an Audi, local media report.
They say one eyewitness may have made a note of the number plate and given it to police.
Brussels Mayor Yvan Mayeur said three men and a woman had been caught up in what he thought was probably a "terrorist act". "It's clearly extremely serious," he was quoted as saying, "and on the Jewish Museum too, which isn't a coincidence".
Eyewitness Alain Sobotik told AFP news agency he had seen two bodies in the lobby of the museum.
One was "a young woman with her head covered in blood", he said. "She was holding a leaflet and looked like a tourist."
Interior Minister Joelle Milquet said everything pointed to an anti-Semitic attack.
Belgium has a Jewish population of some 42,000, about half of whom live in the capital.
Jewish community leader Julien Klener agreed the motive was probably anti-Semitic: "The assumption, and it is an assumption, is that it was someone who didn't try to target the museum but the adjective 'Jewish'".
A number of people were treated for shock after the shooting in the central Sablon area of the city.
Mr Di Rupo expressed his condolences and support for the victims' families.