Leaked documents suggest that French secret services stopped tracking Toulouse gunman Mohamed Merah, despite evidence of his extensive links to jihadists.
Le Monde newspaper says it has seen notes from the domestic intelligence agency DCRI describing his successful efforts to conceal his movements.
The judge investigating the case said he was perplexed by the DCRI decision.
Mohamed Merah killed seven people in March before being shot dead by police.
The victims included three soldiers and four Jewish people.
The leaked papers suggest there was more than just suspicion on the part of the French intelligence services.
Mohamed Merah had been tracked by the security services since 2006.
The report prepared for the French government and leaked to Le Monde cites a DCRI officer raising concerns about the man in March 2011.
The officer said Mohamed Merah rarely left his home and was paranoid and suspicious. He had no internet in his flat, did not appear to have a mobile phone and always used public telephone booths.
Another note, on 26 April 2011, reported that Mohamed Merah was violent to women for having shown disrespect to a Muslim.
The note said he glorified the murder of “Western infidels” in songs he composed, and he was photographed with a knife and Koran. He travelled frequently to the Middle East.
He had a long list of contacts to Islamist movements in the UK, the same leaked document says.
According to Le Monde, Mohamed Merah was last questioned in November 2011 and had great difficulty explaining a visit to Pakistan where he had been training with militants.
Just a week later, the DCRI suddenly stopped monitoring him.
Judge Christophe Teissier said he was surprised by the move.
The judge said Mohamed Merah’s profile was typical of a home-grown threat – he was independent, radicalized quickly, and did everything possible to conceal the support and training he was receiving.
In August, Le Monde said other documents it had seen showed Mohamed Merah had made more than 1,800 calls to over 180 contacts in 20 different countries.
Mohamed Merah was shot dead on 22 March after a huge manhunt culminated in a 32-hour stand-off with police at an apartment in Toulouse.
The Jewish victims included three children murdered at a school.
Mohamed Merah’s rampage, from 11 to 19 March, terrorized the region.