The suspects were believed to have taken orders from Syria, and used a mobile phone and computers to search for locations that officials say might have been potential attack sites. Security was tightened at the headquarters of France's criminal investigations police, reportedly among the locations studied.
The five were arrested in Sunday in Strasbourg and Marseille, and should have been charged or released today.
Authorities used an exceptional anti-terrorism measure to extend their custody up to two more days, according to the Paris prosecutor's office.
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