The Paris prosecutor's office says authorities have found phones, USB flash drives and other equipment during searches of about 80 prison cells of Islamic radicals after terrorist attacks earlier this month.
The prosecutor's office said that investigators are analysing the material found in the searches.
Two of the Islamic extremists behind the attacks earlier this month had met in a French prison. They had also been in contact with prominent imprisoned radical Djamel Beghal, convicted of plotting to blow up the US Embassy in Paris.
Reports said the authorities found a phone in Beghal's cell during the searches. Mobile phones are banned in French prisons, but often circulate among inmates.
Twenty people died in the attacks on a kosher market, newspaper Charlie Hebdo, and police - including the three gunmen.