A French-Algerian man and his accomplice were found guilty of shooting four people in the Jewish Museum of Brussels five years ago, the first verdict against a European who joined Islamic State in Syria and returned to stage terrorist attacks.

A Belgian jury on said Mehdi Nemmouche, a 33-year-old French national of Algerian descent, had shot and killed two Israeli tourists, an employee and a museum volunteer in fewer than two minutes in the May 2014 attack.

His sentence, which could be life in prison, is expected to be read Monday. Jury trials are rare in Belgium, and the 12 jurors deliberated for two days before reaching their verdict after a two-month trial.

Nemmouche’s co-defendant, Nacer Bendrer, a 30-year-old Frenchman of Algerian descent, was convicted of supplying weapons to and helping Nemmouche carry out the attack. Bendrer’s sentence, which could also be life in prison, is due Monday, too.

The two men met in prison in southern France in 2008, where they were serving sentences for robberies. According to the prison director at the time, who testified during the trial, Nemmouche had already been radicalized and had a strong influence on his new acquaintance. Once out of prison, in December 2012, Nemmouche left for Syria and joined Islamic State.

Prosecutors put forward 23 pieces of evidence incriminating Nemmouche as the assailant in the two-month trial at Brussels’ Palace of Justice.

During the trial, two French journalists who had been abducted in Syria by Islamic State, identified Nemmouche as their guard while in captivity. They described him as a “playful sadist” who enjoyed torturing his other victims.

Evidence presented during the trial also exposed Nemmouche as a member of the same Franco-Belgian terror cell that carried out attacks in Paris and Brussels in 2015 and 2016, killing a total of 162 people.

While Nemmouche was in Syria, his partner who dealt with the hostages later turned out to be one of the suicide bombers in the 2016 Brussels airport attack.

In January 2014, while still in Syria, Nemmouche was called from Belgium by one of the coordinators of the Paris attacks, Abdelhamid Abaaoud. After that phone call, the Jewish Museum website was visited from domain names in Syria and Singapore on the dates Nemmouche was in those locations. This suggests that Abaaoud was the one who assigned Nemmouche to carry out the Jewish Museum attack, according to the lawyer representing Belgium’s Jewish organizations.

Following Nemmouche’s arrest, six days after the museum attack, he spoke knowledgeably about the terror cell with other inmates, according to testimony.

In closing statements, Nemmouche claimed he had been framed, but he also said that “if I could change things, I’d change everything.”

Nemmouche and his lawyers maintained his innocence throughout the trial, arguing that he had been framed in a Lebanese-Iranian assassination plot targeting the Israeli tourist couple, Miriam and Emmanuel Riva.

Nemmouche’s defense alleged that he was framed by Lebanese and Iranian agents who carried out the assassination of the Riva couple, who Nemmouche said were Israeli security agents. The lawyers also claimed that the surveillance videos were doctored and that Nemmouche wasn’t at the museum.

The couple’s two daughters traveled from Israel to testify at the trial about the ordeal of being orphaned in their teens. They said their parents, both in their early 50s, had picked Brussels as a weekend destination to celebrate 18 years of marriage. The daughters asked Nemmouche’s lawyers to stop using their parents as part of his defense strategy, said the Riva family lawyer, Marc Libert.

Mr. Libert said Ms. Riva had worked as an accountant for the Israeli intelligence service, Mossad, but never as an operative. Mr. Riva had worked for Nativ, an Israeli agency facilitating the immigration of Jewish people from the former Soviet Union.

The European Jewish Congress said the verdict was a “successful stress test for the rule of law in Belgium.”

The prosecution put forward 23 pieces of evidence incriminating Nemmouche as the assailant, including footprints, traces of gunpowder found on his jacket and six videos claiming the attack with his voice off-camera.

“During my 25-year career, I’ve rarely seen so many incriminating elements that the suspect continues to deny,” said the prosecutor, Bernard Michel.