"We will take the most severe measures, we will do what we must do," Macron said in a televised interview.
The assailant, Ahmed Hanachi, had been arrested for shoplifting two days before the October 1 attack in the eastern city of Lyon.
He was allowed to walk free the following day -- a decision the government's inspectorate general (IGA) said revealed "serious faults" in the handling of foreigners whose papers are not in order.
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People evacuating Marseille's Saint-Charles train station after the knife attack. Photo: AFP
Hanachi, 29, fatally stabbed the two women at Marseille's Saint-Charles train station before being shot dead by police.
The attack was claimed by the Islamic State group, but French investigators have not found any evidence linking the attack to the jihadists.
But he was known to the police for drug as well as alcohol problems and had a history of petty crime, using seven aliases.