Mehdi Nemmouche, the French-Algerian jihadist believed to have shot dead four people at the Jewish Museum in Brussels on May 24, was transferred to Belgium on Tuesday to stand trial, having been held in Paris for almost two months.
Nemmouche, 29, is thought to have fled to Amsterdam after the massacre. He was eventually arrested on May 30 on a bus in Marseilles.
Whether Nemmouche was found by pure chance or as a result of a coordinated European police investigation is not clear. At any rate, he was carrying weapons similar to those used in Brussels at the time of his arrest.
Ever since his arrest in Marseilles, Nemmouche has refused to answer questions from the French judges, presumably on the advice of his French-Cameroonian lawyer, Apolin Pepiezep.
Likewise, he has declined, so far, to co-operate with the Belgian judges who are now in charge of the case.
One reason may be his fear of being extradited to Israel. Two of the Jewish Museum victims were Israeli citizens. Mr Pepiezep argued that police leaks to the press were valid reasons for Nemmouche's refusal to speak.
Born out of wedlock in Roubaix, northern France, to a mentally disturbed mother of Algerian origin and an unknown father, Nemmouche spent his childhood between his Algerian relatives and a French foster family.
He engaged in petty thievery at 15, and soon got mired in violence and crime. Convicted on several charges, he spent five years in jail, from 2007 to 2012.
He opted for a militant Islamic lifestyle while in jail - not unusual among Muslim criminals. Once set free, he travelled to Turkey, Malaysia and Thailand in 2013. It is assumed that he spent one year in Syria with Islamic State militias.
According to French intelligence and security agencies, several hundred young men with a similar background or profile, and some young women as well, have been trained in Syria over the past two years and have returned to Europe. They are seen as a major security risk.