Viktor Baturin, the brother-in-law of former Moscow mayor Yury Luzhkov, is being investigated for possible tax evasion, prosecutors said on Friday.
The investigation was opened on November 8, Investigations Committee spokesman Vladimir Markin said.
Baturin is suspected of failing to report 178 million rubles (about $6 million) of personal income.
He faces up to three years in prison if convicted.
On Wednesday a Moscow court sanctioned Baturin’s arrest on charges of attempted fraud.
Baturin, who is already serving a suspended sentence, was detained in the Russian capital on Monday.
Investigators allege he demanded payment of 10.8 million rubles ($344,000) from his billionaire sister's company Inteko under a fake promissory note.
In early June this year, Baturin, the brother of property construction tycoon Yelena Baturina, received a suspended three-year jail term for fraud. He was found guilty of selling one property in downtown Moscow to two men for $857,000 and $1.5 million within the space of one month in 2008.
The Presnensky district court also fined Baturin $10,800 in damages.
