The man who stabbed two young women to death in Marseille in an attack claimed by the Islamic State group was arrested days earlier for shoplifting but let go because a key official was absent, sources close to the probe said Monday.
Interior Minister Gerard Collomb has given investigators a week to determine why police in the eastern city of Lyon let the man go after arresting him on Friday, the interior ministry said in a statement.
French prosecutors said the killer, who was shot dead by anti-terror troops after Sunday's attack outside the southern city's main train station, used seven different identities.
Authorities said he had used a Tunisian passport last week under the name of a 29-year-old with the first name Ahmed.
Sources close to the probe said that in addition to the absence of the Lyon official who had authority to sign a detention and deportation order, there was a lack of space in the jail.
Investigators are seeking to confirm the man's identity as the attacker -- who had a history of petty crime but was not on a jihadist watch list -- used seven aliases, anti-terror prosecutor Francois Molins told reporters in Paris.
Molins said "the method of the attacker" was in line with calls from IS to followers in the West to strike vulnerable targets, such as train stations.
The IS group's propaganda agency Amaq claimed the killer was one of its "soldiers", though a source close to the investigation told AFP no solid evidence linked him to the group.
The attack in France's second-biggest city followed a string of stabbings around Europe claimed by or blamed on Islamist radicals.
The man killed two 20-year-old cousins, one of them a "brilliant" medical student who was studying in the city, according to Aix-Marseille University chief Yvon Berland.
Her cousin, a nursing student from Lyon, was visiting her for the weekend.
Molins confirmed that witnesses heard the attacker shout "Allahu Akbar" (God is greatest) as he lunged at the women with a 20-centimetre (eight-inch) knife before threatening soldiers, who shot him dead.
The attacker's fingerprints showed he had had seven brushes with the law since 2005 including the shoplifting arrest on Friday.
He presented the Tunisian passport to police, saying he was divorced, used "hard drugs", and had no fixed address.
French President Emmanuel Macron tweeted Sunday that he was "deeply angered by this barbaric act".
The attack comes as parliament prepares Tuesday to vote on a controversial anti-terror bill that transfers some of the exceptional powers granted to police under a 22-month-old state of emergency into national law.
France has been under a state of emergency since the IS gun and bomb attacks in Paris in November 2015 - part of a string of jihadist assaults that have left more than 240 people dead over the past two years.
But critics warn that making parts of the state of emergency permanent would give police too much free rein, undermining freedoms in a country seen as a beacon for human rights.
Knives have been the weapon of choice in a string of smaller-scale attacks in recent months, mainly targeting troops from the 7,000-strong Sentinelle anti-terror force set up to patrol the streets and vulnerable sites such as stations and tourist attractions.
In most cases, the attackers were shot dead at the start of their rampage, before they could kill others.
The Marseille attack came only days after IS released a recording of what it said was its leader Abu Bakr al-Baghdadi urging his followers to strike their enemies in the West.
The French government has deployed troops and its air force to the Middle East and is a leading partner in the US-led international coalition fighting IS in Iraq and Syria.