The brother of Toulouse killer Mohamed Merah appeared before a judge on Sunday charged with criminal conspiracy.
Merah claimed responsibility for the killings of three paratroopers, three schoolchildren and a rabbi during a stand-off with police that ended with his death on Thursday.
But the Paris prosecutor's office has opened a judicial inquiry into whether his older brother Abdelkader was complicit in preparing or planning the attacks.
Police union spokesman Christophe Crepin said detectives believed Abdelkader Merah had "furnished means and worked as an accomplice."
They were suspicious of how Mohamed Merah had gained access to a large weapons arsenal including an Uzi submachine gun and had been able to rent a car despite having no clear source of income.
Intelligence chief Ange Mancini said that during the standoff, Merah told police he had bought the weapons for about €20,000 (£16,700), having raised the cash through muggings and burglaries. But Mr Mancini added that, on examination, "the weapons will talk."
Abdelkader Merah was questioned by Toulouse police several years ago over alleged links to a network which sent young French Muslims to Iraq, but no charges were ever filed against him. His girlfriend, who was briefly arrested, has now been released as has the brothers' mother.
After a brief hiatus in presidential campaigning as France reeled from last week's shootings President Nicolas Sarkozy tried to revive his flagging poll numbers by attacking immigrants, declaring on Saturday that he would pull France out of Europe's border-free Schengen zone if it did not clamp down on immigration. Mr Sarkozy proposed that the zone should choose a president and a "governing body" that could punish countries that failed to "respect its rules."
Poll results issued last week showed that Mr Sarkozy was on course to receive 28 per cent of the vote, behind Socialist Party candidate Francois Hollande on 29.5 per cent. Jean-Luc Mellenchon of the Left Front, which includes the French Communist Party, was on 14 per cent, pushing the far-right National Front into fourth place on 13 per cent.
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