Anis Hanachi, whom French investigators allege is a former jihadist fighter in Iraq and Syria, was extradited from Italy on Thursday.
According to Italian anti-terrorism chief Lamberto Giannini, French investigators are looking into whether Anis "indoctrinated... Ahmed and caused his radicalisation".
He was arrested in the northern Italian town of Ferrara six days after Ahmed Hanachi, a 29-year-old Tunisian, fatally stabbed the two women outside Marseille's main rail station on October 1 before being shot dead by police.
The Islamic State group claimed responsibility, but French investigators have found no evidence linking the attack to the jihadists.
Anis was charged Friday with "association with criminal terrorists seeking to commit crimes" and detained, judicial sources said.
Ahmed was known to police for drug and alcohol problems with a history of petty crime. He was not on a jihadist watch list.
Two days before the attack, he was arrested for shoplifting in the eastern city of Lyon but was allowed to walk free the following day.
The French government's inspectorate general (IGA) said that decision revealed "serious faults" in the system for dealing with foreigners whose papers are not in order.