In an exceptional move, French anti-terrorism authorities are keeping five men in custody without charge in an investigation into a new plot to attack France.

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 Thursday. Authorities used an exceptional anti-terrorism measure to extend their custody up to two more days, according to the Paris prosecutor's office.

France remains under a state of emergency imposed after Islamic State attacks in Paris a year ago that killed 130.