The mother of a French lawyer murdered at the Bataclan concert hall in Paris has accused Belgium of negligence and vowed to take her case to court.
Many of the 13 November jihadists had a Belgian background.
Nadine Ribet-Reinhart believes the authorities there should have done far more to act against them.
“This is called inaction, and in the wake of such inaction are decimated families and children who will never be born,” she said.
Anti-corruption lawyer Valentin Ribet, 26, was one of 130 people murdered in the series of co-ordinated attacks carried out by Islamists. His girlfriend Eva was also shot but survived.
Much of the planning for the attacks is thought to have taken place in Belgium and authorities there said last week they had identified three hideouts used by the Paris attackers in the hours before they travelled to France on 13 November.
Many of the militants had Belgian-Moroccan backgrounds and were linked to the Molenbeek area of Brussels. Suspected ringleader Abdelhamid Abaaoud grew up in the district and French officials say he was linked to four foiled attacks, including the Thalys train attack in northern France last August.
“Everybody now knows the name of Molenbeek,” Ms Ribet-Reinhart told BFM-TV. “What did the state of Belgium do beforehand? Trials in absentia? Parades of suspects?”
“They could and should have stopped 10 terrorists being present with complete impunity on French territory that night; people who travelled around at will on the metro and in their cars with their mobile phones.”
What sort of action the victims’ families could take is unclear.
Belgian lawyer Johan Platteau told Belgian radio on Tuesday that the state could be held responsible, but there would have to be concrete evidence of negligence, in the form of proof that it had information and failed to act on it. The mother would have to provide the evidence, he added.
Ms Ribet-Reinhart said she planned to take action against Belgium for the sake of justice and for her son. “We, with his family and (her son’s fiancee) Eva, will provide the means and go as far as it takes.”
Other Belgians named as attackers include Chakib Akrouh, who was killed with Abaaoud in a Paris police raid in Seine Saint-Denis, and Brahim Abdeslam, who blew himself up after the gun attacks on bars and restaurants.
Stade de France bomber Bilal Hadfi came from France but had been living in the Brussels area.
Brahim Abdeslam’s brother Saleh was able to return to Belgium after the attacks and is still on the run. For several days last November, Brussels was put on high terror alert amid fears of Paris-style attacks.
Another Belgian of Moroccan descent was arrested in Morocco last Friday. Gelel Attar is said to have lived in Molenbeek and had direct links with Abaaoud and Akrouh. Like many of the suspects linked to Belgium he is said to have joined jihadist rebels in Syria.