The girlfriend of the man who carried out last week’s attack in south-west France was known to the security services, local media report.
A source close to the investigation told the AFP news agency the 18-year-old French woman, who is in custody, showed “signs of radicalisation”.
Four people were killed and 15 injured in the attack on 23 March.
The gunman, 25-year-old Redouane Lakdim, was on an extremist watch list but it was decided he was not a threat.
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He was shot dead by police after killing and injuring a number of people in separate incidents, including taking hostages at a supermarket in the town of Trèbes.
Tributes have been paid to the victims, including a French gendarme who was killed after swapping places with one of the hostages.
According to French-language radio station RMC, the attacker’s girlfriend is a convert to Islam who has been known to security services for at least a year. She has not been named but remains in police custody.
A 17-year-old said to have been a friend of the attacker is also being held.
Lakdim, who was born in Morocco and became a French citizen in 2004, was a petty criminal before he was flagged as a potential security threat in 2014.
During the attack, he pledged allegiance to the Islamic State group and is said to have demanded the release of Salah Abdeslam, the most important surviving suspect in the 13 November 2015 attacks in Paris, which killed 130 people.
Prosecutor Francois Molins said Lakdim had been on an extremist watch-list due to “his radicalisation and his links with the Salafist movement”, a hard-line offshoot of Sunni Islam. However, there had been no indication he would carry out an attack.