The alleged killer of four at the Brussels Jewish Museum in May beat Western hostages while fighting with Isis in Syria, according to a former prisoner of the terror group.
French journalist Nicolas Henin, who was held by Isis for six months until his release in April, told reporters this week that Nemmouche had beaten him and other hostages - including the murdered US journalists James Foley and Steven Sotloff - in a hospital in Aleppo.
Mr Henin said that he had decided to come forward following the beheadings of Mr Foley and Mr Sotloff.
"I encountered Nemmouche several times during my six months of captivity in Syria," Mr Henin told reporters.
Nemmouche would boast of his sadism, telling how he had raped and murdered a young mother before beheading her baby, he said.
According to Mr Henin, Nemmouche also told his prisoners that he was planning to carry out a terror attack on the French military parade in Paris on Bastille Day which, he said, would be "five times what Merah did".