Security has been stepped up at Jewish sites across Belgium after a gunman shot dead three people at the Jewish Museum in the capital Brussels.
A man who was detained by police shortly after the attack has been released without charge. A manhunt has been launched for a different suspect.
An Israeli couple in their 50s and a French female employee of the museum were killed in the attack.
A fourth victim, a Belgian employee, remains in a critical condition.
The Belgian prosecutor's office said the victims were struck by bullets in the face or throat, in what Brussels Mayor Yvan Mayeur said was probably a "terrorist act".
Deputy prosecutor Ine Van Wymersch said the gunman "probably acted alone, was armed and well prepared," adding that "all options are still open" regarding a motive.
She appealed to the public to help identify the gunman and said that images taken at the time of the attack would soon be released.
Israeli Prime Minister Benjamin Netanyahu issued a statement strongly condemning the killings which he said were "the result of endless incitement against the Jews and their state".
French President Francois Hollande said there was no doubt about the "anti-Semitic character" of the attack.
His comment came hours after two Jewish men were beaten as they were leaving a synagogue in Creteil, 13km (8.1 miles) south-east of Paris on Saturday night.
At the scene: Laurence Peter, BBC News, Brussels
People are laying flowers at the entrance to the Jewish Museum, and some candles are burning there too. One old man came up on a mobility scooter to place his bouquet.
The museum is in a narrow cobbled street, next to an antiques shop, in the picturesque old Sablon quarter. A police van is parked at the scene now, as TV crews and grim-faced locals mill around.
A Catholic priest at the scene said it was a "terrible" attack and he was praying for the victims.
While the authorities cannot yet confirm the gunman's motives, Joel Rubinfeld, President of the Belgian League against Anti-Semitism, said he had no doubt this was an anti-Semitic attack.
"Yesterday a cold-blooded killer went into the streets with the clear idea of killing Jews," he told the BBC.
Belgian Foreign Minister Didier Reynders, who was one of the first people to arrive at the scene, said: "You cannot help think that when we see a Jewish museum, you think of an anti-Semitic act. But the investigation will have to show the causes."
Belgium has a Jewish population of some 42,000, about half of whom live in the capital.
Belgium's Prime Minister Elio Di Rupo said "everything has been mobilised that can be mobilised" to find the killer.
It is believed the shooter parked a car outside before entering the museum, firing and leaving the scene quickly.
The attack happened at around 15:50 local time (13:50 GMT) on Saturday in the busy Sablon area of Brussels, which was hosting a three-day jazz festival.
One person was detained after he drove away from the museum around the time of the attack, but Belgian police say the man has now been released and is being treated as a witness.
A second suspect left the area on foot and security camera footage is being studied to try to identify the person.