Increasingly damning evidence suggested France was aware Mr Mehar posed a threat.
French intelligence had previously alerted security services in Spain that Mr Merah was planning to travel to the Costa Brava to attend a meeting of Islamist activists, suggesting they considered him dangerous.
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The warning said that he was probably on his way to attend a suspected Salafist congress.
The prosecutor said that he had been arrested by Afghan police in 2010 in Kandahar and handed over to US army troops, who put him on a flight back to France – a claim US military did not confirm on Thursday.
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