JERUSALEM—Four people, including an Israeli couple, were killed in an attack inside the Jewish Museum of Belgium in Brussels late Saturday afternoon. Security was been tightened at Jewish institutions around the country.

The shooting was called the worst anti-Semitic attack in Belgium since World War II and came a day before Belgians were to cast their ballots in regional, federal and European elections on Sunday.

The gunman was still at large Monday, and Belgium’s interior minister Joëlle Milquet said that “maximum security has been deployed around places frequented by the Jewish community.” There were no special security measures in place at the museum prior to the attack, which may have been selected as an easy target, she noted.

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Moniquet told a news conference that heightened security was necessary because “the perpetrators presumably knew they were on a ‘no-way-out’ operation. They are working under the assumption that they will be caught within days, and therefore have a motivation to maximize the attack by striking again if capable,” she warned.

According to earlier reports, a lone gunman double-parked an Audi sedan outside the museum at about 4 p.m., ran into the museum, and quickly opened fire before returning to the car and driving away. Police later said that he may have fled on foot.

Police arrested a suspect within hours of the shooting in a car that matched the description and license-plate number given by eyewitnesses, and is considered a witness, according to local press reports.

The Times of Israel web site reported that the couple was identified late Sunday morning as Tel Aviv residents Mira and Emmanuel Riva, who were on an organized, private walking tour of Brussels when the shooting took place. It also said that Israel’s Foreign Ministry would fly the bodies home for burial. The third fatality was a 23-year-old female museum employee. A fourth victim, Alexandre Strens, died on Sunday after being rushed to the hospital after the attack.

Didier Reynders, Deputy Prime Minister and Foreign Affairs Minister of Belgium, told the local media that he was near the museum when he heard shots ring out and ran to the scene.

“You can’t help thinking when you see a Jewish museum to think about an anti-Semitic act,” he said. “But the investigation will tell more.”