French prosecutors have opened an investigation into the leak of large quantities of hacked data from Emmanuel Macron's campaign two days before Sunday's presidential election, which the centrist won, a judicial source said on Tuesday.
Mr Macron's team said a "massive" hack had dumped emails, documents and campaign financing information online just before campaigning ended on Friday and France entered a quiet period which forbade politicians from commenting on the leak.
French authorities worked to keep the hack from influencing the outcome of the election, with the electoral commission warning the media on Saturday that it could be a criminal offence to republish the data.
Prosecutors are investigating "entry into an automated data system and violating the secrecy of correspondence", the judicial source said.
The probe will be carried out by a Paris police unit specialising in computer crimes.
The French government cybersecurity agency ANSSI also said on Tuesday that the presidential election commission had asked it to provide technical expertise on the hacking attack.
Macron, long the front-runner, emerged the comfortable winner of Sunday's election, winning 66 percent of the vote to 34 percent for far-right leader Marine Le Pen.
Macron's party said the leaked documents dealt with the normal operations of his campaign. It said the hackers had mixed false documents with authentic ones to "sow doubt and disinformation".
The US cyber intelligence firm Flashpoint told Reuters late on Friday that an initial review of the Macron leaks indicated that APT 28, a group tied to the GRU, the Russian military intelligence unit, may be behind the leak, though evidence was not yet conclusive.
The Kremlin has previously denied being behind media and internet attacks on Macron's campaign.
Russia also denied interfering in last year's US election campaign after US intelligence agencies accused Russian President Vladimir Putin in January of ordering an effort to help Republican Donald Trump's electoral chances by discrediting Democrat Hillary Clinton.
The German government has warned political parties to step up their defences against hacking after last week's leak of emails from Macron's campaign.