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, reports the BBC.
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.
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.