The probe against Didier Julia, a deputy in President Jacques Chirac's ruling UMP party, as well as his two assistants in the abortive operation, Philippe Brett and Philippe Evanno, could lead to a formal investigation if evidence of wrongdoing is uncovered.
One of the freed reporters, Le Figaro correspondent Georges Malbrunot, said on his arrival in Paris a week ago that he was "scandalised by Julia's behaviour – playing with the lives of two compatriots. It is beneath contempt".
He said his life and that of fellow hostage Christian Chesnot of Radio France Internationale, were put at risk by Julia's claims that he had secured their release in September but failed at the last minute because of US interference.
A group calling itself the Islamic Army in Iraq said it decided to release the two journalists on 21 December after keeping them hostages for four months because it had determined they were not US spies and because of France's opposition to the US-led war in Iraq.
The two French reporters were
held hostage for four months
The French government has insisted no ransom was paid for the men’s release, but has not given details of how they were freed.
French legal authorities on Wednesday said top anti-terrorist judge Jean-Louis Bruguiere and Marie-Antoinette Houyvet would head the investigation against Julia and his aides.
They were looking into suspicions that the three dealt in intelligence with a foreign country that could "undermine the fundamental interests" of France, notably in terms of diplomacy and in the safety of its population.
Brett and Evanno were arrested on Monday.
Julia, who has parliamentary immunity from criminal prosecution, may be summoned later to answer questions.
He has strenuously denied any wrongdoing and claimed he was the victim of a political vendetta by Foreign Minister Michel Barnier.