The French finance ministry confirmed Monday it had come under cyber attack in December from hackers targetting G20 documents.
Budget Minister Francois Baron said an investigation was underway into the origin of the attacks.
"We have leads," unconfirmed at this stage, Baron said.
Patrick Pailloux, directeur general of the French National Agency for Information Technology Security, told Paris-Match magazine hackers were after "documents related to the French presidency of the G20 and to international economic affairs."
"The actors were determined professionals and organised. It is the first attack of this size and scale against the French state," he added.
More than 150 ministry computers had been hacked and numerous documents pirated. Individuals were not targetted in the attack.
Bercy has filed an official complaint with the French courts and the French secret service has taken up the case.