BRUSSELS — Belgium launched a nationwide hunt Sunday for a lone gunman who walked into the Jewish Museum, unpacked a Kalashnikov rifle, killed at least three people with a short burst of fire, packed up and briskly walked away.

Saturday’s attack led Belgian officials to immediately raise anti-terror measures and increase the protection for Jewish sites for what was widely seen as an anti-Semitic attack. It happened on the eve of Belgium’s nationwide election and as voters across the continent picked a new European Parliament.

Video of the attack showed an athletic man with cap walking determinedly into the small Jewish Museum in the swank Sablon area, clearly bent on quickly carrying out an attack.

A government official had said a fourth victim, a Belgian man, died Sunday afternoon, but neither the interior ministry nor the prosecutor’s office could officially confirm it late Sunday. The victim has been in a critical condition since he was shot.

“We call on the whole population to help identify this person,” deputy prosecutor Ine Van Wymersch said Sunday before three videos and still photos of the attack were posted on the federal police website. None has a clear view of the man’s face.

She said the gunman who killed an Israeli tourist couple and a French woman “probably acted alone.”

Police detained one suspect Saturday but he was released and is now considered a witness.

“The pain is excruciating as one can imagine,” said Rabbi Michoel Rosenblum, director of the EU Jewish Buildings in Brussels. “The fact that the gunman is at large is very concerning and I hope we will be able to take care of that ASAP.”

On the heels of the Brussels attack, two Jewish men were attacked as they left a synagogue in the Paris area late Saturday.