Less than a week after the men were sent back to France, anti-terrorism judges placed Nizar Sassi, Mourad Benchellali, Imad Kanouni, and Brahim Yadel under formal investigation - one step short of laying formal charges under French law.
The inquiry was opened on grounds of suspected "association with criminals engaged in a terrorist enterprise". Sassi and Benchellali are also being investigated for the possession and use of forged passports, judicial sources said.
Anti-terrorism judges Jean-Louis Bruguiere and Jean-Francois Ricard on Saturday called for the temporary detention of the four men, who were sent back to France on Tuesday after more than two years at the US military jail at Guantanamo in Cuba.
Still wearing the white T-shirts, jeans and trainers provided by US authorities on their release, the four were transferred to prisons near Paris after another judge confirmed their detention early on Sunday.
William Bourdon, a lawyer for Sassi and Benchellali, said the detention was unjustified.