A judicial source told AFP the investigation had been opened into Rifaat al-Assad, the brother of Bashar al-Assad's father Hafez, after a criminal complaint filed on September 13.
The complaint, by anti-corruption groups Sherpa and Transparency International, alleges the 76-year-old illegally acquired "extraordinary wealth" in France through corrupt schemes and embezzlement.
Once a stalwart of the Syrian authorities, Rifaat al-Assad broke with his brother's government in 1984 and reportedly has no links with the current regime, which is fighting in a civil conflict that has left more than 110,000 dead since it began in March 2011.
The massacre in the town of Hama, by troops allegedly under Rifaat al-Assad's command, left between 10,000 and 25,000 dead.
More From This Section
Rifaat al-Assad has denied any involvement and in 2011 dismissed allegations he was behind the killings as "a myth".
The criminal complaint accuses Rifaat al-Assad of acquiring wealth "in the billions of euros" through corruption, embezzlement of public funds, misuse of corporate assets and other crimes, noting that he had "no known professional activity".
"It is obvious that only an examining magistrate has the necessary authority to deal with offences of such a complex and international nature," he told AFP, adding that a magistrate would also have more power to seize assets.
French media have reported that Rifaat al-Assad's holdings include a mansion and several dozen apartments in Paris, with newspaper Le Monde estimating the total value of his estate in France at USD 215 million.
Le Monde reported earlier this year that the potential sale of one of his properties -- a mansion on the prestigious Avenue Foch -- fell through after potential Russian buyers offered only 70 million euros.