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.
More top news
Southern areas windy and very mild.
Nobel-prize-winning schoolgirl Malala Yousafzai has said she is a proud Brummie, as a new portrait of her was unveiled in her adopted home.
The Filipino girl group lost out to Lauren Murray in the sing off after receiving the lowest number of votes.