PARIS — Citing the risk of imminent threat, French anti-terrorism authorities are keeping five men in custody without charge in an investigation into a suspected new plot to attack France.

The suspects were believed to have taken orders from Syria, and used a cellphone 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 early Thursday. Authorities used an exceptional recent anti-terrorism measure to extend their custody up to two more days, according to a judicial official.

The official said the exceptional move was based on the threat of an imminent terrorist act and the need for further information from international partners.

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A security official said investigators have determined that the suspects used the phone and computers to search locations, including an amusement park — but said it was too early to tell whether these were attack targets as such. Both officials spoke on condition of anonymity to be able to discuss ongoing investigations.

France remains under a state of emergency imposed after ISIS attacks in Paris in November 2015 that killed 130 people.