A French citizen suspected of being a member of Islamic State has been convicted over a terror attack carried out in Belgium in 2014.

Mehdi Nemmouche was found guilty of terrorist murder over the shootings of four people at a Jewish museum in Brussels – the first attack carried out in Europe by a jihadist returning from fighting in Syria.

An Israeli couple, Miriam and Emmanuel Riva, a young Belgian employee of the museum, Alexandre Strens, and a French volunteer, Dominique Sabrier, all died in the attack.

Nemmouche, who appeared in court with his beard trimmed and wearing a navy blue jumper on Thursday, now faces a life sentence. The 33-year-old showed no emotion and stared into space as the verdict was delivered. Sentencing could be as early as Friday.

Nemmouche was found to have killed the four victims in cold blood in less than 90 seconds in the antisemitic attack on 24 May 2014. He denied the accusation, telling the court he had been “tricked”.

Defence lawyers argued that Nemmouche was not to blame for the killings, but that he was caught up in a plot targeting the Israeli intelligence agency Mossad.

The legal argument had centred on the Rivas, the first two of the four people killed. According to the defence, the museum shooting was not the work of Isis, and the Israeli couple were in fact Mossad agents murdered by another man who had hunted them down.

The Riva family’s lawyers have furiously rejected the theory, and said attempts to pass off the tourists as secret agents were “an absolute scandal”.

“Let’s stop the joking,” prosecutor Yves Moreau told the court this week, describing the defence’s arguments as “complete nonsense”.

Miriam Riva worked for Mossad but, as an accountant, she was not operational, said the investigating judges, who travelled to Israel during their investigation.

Yohan Benizri, the head of Belgium’s Coordinating Committee of Jewish Organisations, denounced what he called a “nauseating conspiracy theory”.

The 12 jurors also found fellow Frenchman Nacer Bendrer, 30, who was accused of supplying the weapons, to be the co-author of the attack.

Six days after the massacre, Nemmouche was arrested in the French city of Marseille in possession of a revolver and a Kalashnikov-type assault rifle.

At the trial, Bendrer admitted that Nemmouche had asked him for a Kalashnikov when he came to Brussels in early April, but claimed he never delivered it.

Upon arrest, Nemmouche possessed a nylon jacket with gunshot residue, as well as a computer with six videos claiming the attack and featuring an off-camera voiceover thought to be by him.

The jury also heard testimony last month from two French journalists who were held hostage by Isis in Syria and identified Nemmouche as one of their captors.

The journalists described him as deeply antisemitic, sadistic and full of hatred.

Nemmouche faces separate charges in France for his role in keeping the reporters in captivity

Agence France-Presse in Brussels contributed to this report.