Nemmouche’s detention was prolonged 24 hours on Tuesday, taking it over the usual four days in terror cases – an exceptional measure that is rarely resorted to.
No reason was officially given but the law allows for extended detention in the case of “imminent danger” of a new attack or for the sake of “international cooperation”.
Nemmouche, who is believed to be the gunman who killed three people and left a fourth in critical condition in the 24 May attack, has said nothing to interrogators and has refused to be taken out of his cell.
He is expected to appear before a magistrate on Wednesday to be notified that he is the object of a European arrest warrant and then detained awaiting extradition to Belgium, a move that he is reported not to oppose.
Investigators want to know why he had fled to Marseille, possibly to seek fellow former prison inmates to help him on the run, plan a further attack or go to Algeria, where his family comes from.
Ballistic tests in Belgium should establish whether the weapons he had in his luggage were the ones used in the Brussels attack.
A 23-year-old French national, Cédric Vuillemin, was arrested in the Belgian capital on Tuesday in connection with networks taking Islamist fighters to Syria.
Police said there was no connection with the attack on the Jewish Museum.
Nemmouche spent over a year with Islamist militias in Syria, French intelligence says.
Valls on Tuesday said that anti-terror police have upped their estimate of French nationals who have fought or intend to fight in Syria to 800.
"We have never before faced a challenge of this kind," Valls told BFMTV. "It is without any doubt the most serious threat we face. We have to ensure the surveillance of hundreds and hundreds of French or European individuals who are today fighting in Syria."