The 29-year-old Frenchman arrested over the fatal shooting of three people at Brussels' Jewish Museum was radicalised during the last of his five stays in jail in France, a prosecutor said.
"During his last stay in jail he was noticed for extremist (Islamic) proselytism," Paris prosecutor Francois Molins said.
He said that the suspect, Mehdi Nemmouche, travelled to Syria only three weeks after he was freed in December 2012.
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One of the attackers who murdered a priest in a French church during morning mass in Normandy has been named by prosecutors.
Father Jacques Hamel has been described as man who dedicated his life to his parishioners and still regularly led mass, even at 86.
Rain will spread southeastwards through N. Ireland, Wales and England overnight.