French anti-terror judges ordered the detention on Saturday of a man on suspicion he aided an Al-Qaida-inspired gunman prepare for a shooting spree last year, a judicial source said.
Mohamed Merah killed four Jews and three soldiers in and around the southern city of Toulouse in March 2012 before he was shot dead by police.
Anti-terror judges have put the unnamed 25-year-old detained man under formal investigation to determine whether he helped Merah steal a scooter that was used in the shootings.
Merah's brother Abdelkader has also been in detention since March last year on suspicion of complicity in terrorism, murder and theft. He denies being an accomplice in the killings.
Under French anti-terror laws suspects can be held for up to four without charges while an investigation is carried out.
The attack was the worst on French soil since 1995 bombings on the France's underground train network by Algeria's Armed Islamic Group (GIA) that killed eight and wounded scores.
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