Six days after the attack, on 30 May 2014, Mehdi Nemmouche, a 29-year-old French national of Algerian origin, was arrested during a routine drugs check in Marseille, France, when he was found to be carrying weapons identical to those used in the shooting.

He next took an assault rifle out of a bag to shoot a French female volunteer at her desk in a nearby room, after which he fled the scene on foot and disappeared into the crowd.

Belgian police appealed to the public to help identify him, releasing CCTV from the museum which showed his image, partly obscured.

[9] Mehdi Nemmouche (Arabic: مهدي نموش; born 17 April 1985), a 29-year-old French citizen, was arrested on 30 May 2014 at the Saint-Charles railway/bus station in Marseille during a random drugs search when he got off a coach arriving from Amsterdam via Brussels.

[17] The Belgian federal prosecutor's office, which was responsible for the investigation, requested the council chamber of the tribunal of first instance of Brussels to start indictment proceedings for Nemmouche and Bendrer at the end of 2017.

[20][21] In January 2019, the trial of Mehdi Nemmouche and Nacer Bendrer, who was accused of being an accomplice, started before the court of assizes of Brussels.

A few weeks before the actual start of the trial, a preliminary hearing took place on 20 December 2018, during which a list of about 120 witnesses, which were to be heard in the case, was decided on.

[23] On 10 January 2019, the actual trial started with the prosecution reading the 195-page "act of accusation" to the court and jury, wherein it described the charges brought against the two men and the evidence gathered during the investigation.

They focused on the perceived lack of DNA traces on some of the material evidence, and accused the Mossad (foreign intelligence agency of Israel) of having set up Nemmouche.

[27] Over the next weeks, the court heard the testimonies of the firsthand witnesses, first responders, investigators, experts, and family members of both the victims and the accused.

An important testimony was that of two French journalists, Didier François and Nicolas Hénin, who had been held hostage from 2013 to 2014 in Syria by ISIS operatives.

Whilst the wife, Miriam Riva, did work for the Mossad until her retirement, she did so as an accountant and never came into contact with any intelligence operations, they stated.

Amongst other things, they mentioned a lack of footage from security cameras other than those of the museum, accused the two French journalists who testified during the trial of lying, and suggested Nemmouche's DNA material was planted at the scene after the attack.

According to the defence, Nemmouche was recruited by Iranian and Lebanese security services after being released from prison in 2012, and went to Syria to infiltrate ISIS on their behalf, after which he was sent to Brussels.

The evidence against Nemmouche that convinced the jury included the fact he was carrying the weapons used in the attack with him when he was arrested in Marseille, together with the clothing the perpetrator worn as seen on the security footage from the museum.

Lastly, on a computer belonging to Najim Laachraoui, one of the suicide bombers of the 2016 Brussels bombings, conversations were discovered wherein Nemmouche was described as a "brother", and plans were discussed to liberate him from his incarceration.

The verdict stated Bendrer was aware of Nemmouche's radicalisation and adherence to Islamic extremist beliefs from when they got to know each other in 2009, during their incarceration in Salon-de-Provence, France.

However, the court also upheld some mitigating circumstances, such as Bendrer having steered clear of any Islamic extremism since his arrest, and him having shown empathy and regret for the victims.

[46] Subsequently, on 22 October 2019 the court of assizes awarded 985,000 euro in damages to the victims' next of kin and the Jewish Museum, to be paid by both convicts.

[48] Minister of the Interior, Joëlle Milquet speculated that anti-Semitism might be behind the shootings and Brussels Mayor Yvan Mayeur described them as a likely terrorist attack.

[49] Belgian politician Mischaël Modrikamen, leader of a small conservative party and himself a member of Belgium's Jewish community, said "Sadly, however, the actual attack comes as no surprise to us after years of living in an atmosphere of rampant anti-Semitism that often leads to violence.

"[50] Joel Rubinfeld of the Belgian League Against Antisemitism described the act as "the inevitable result of a climate that distills hatred... it will be necessary to use all legal means to silence the preachers of this hate who are responsible for spreading this virus of anti-Jewish hatred", specifically mentioning the anti-Zionist and Holocaust-denying representative, Laurent Louis, and controversial French comedian Dieudonné M'bala M'bala.

[51] Louis denied Rubinfeld's accusations, suggesting that the attack could have been a false flag operation seeking to discredit him and his political party, Debout Les Belges!

In a statement the league said, "These crimes with racist and anti-Semitic accents are unfortunately likely to reverse in our country the efforts of all those who, on a daily basis, are working for a society where everyone, regardless of his religious and philosophical beliefs, can live in dignity and respect.

[54] Pope Francis, who was visiting the Middle East at the time of the attacks, said he was deeply saddened by the killings in "this criminal act of anti-Semitic hatred".

"[56] Israeli President Shimon Peres also called upon European leaders to act against "any form of anti-Semitism" which he said was "rearing its head across the continent".

[58] Italian President Giorgio Napolitano recalled "the needing to keep in guard and be ready to wear out every regurgitation of antisemitism" and declared "I am always close to the world of Jewish communities yet again harshly hit".

[60] Carl Bildt, Swedish Minister for Foreign Affairs, wrote on Twitter: "Despicable attack on the Jewish Museum in Brussels.

"[61] Birgitta Ohlsson, Minister for EU Affairs, also wrote on Twitter: "I'm shocked about the anti-Semitic attack at the Jewish Museum in Brussels just one Day before the elections to the European Parliament.

"[66] World Jewish Congress President Ronald S. Lauder reacted with shock, saying "Two years after Toulouse, and on the eve of the European elections, this despicable attack is yet another terrible reminder of the kind of threats Europe's Jews are currently facing.