Investigators on Monday said they are running a check to determine whether Left Front leader Sergei Udaltsov, currently under house arrest over accusations that he organized mass riots, used a SIM card issued to State Duma Deputy Ilya Ponomaryov.
Udaltsov's use of the SIM card could lead to new charges against the outspoken opposition leader.
Under the conditions of his house arrest, Udaltsov is prohibited until April 6 from communicating by any means with anyone except his family, lawyers, investigators or prison officials, and Ponomaryov's SIM card is protected from wiretapping because of his position as a Duma deputy.
Investigators' announcement follows a December report in the Kremlin-friendly Izvestia daily that Udaltsov racked up a 107,000 ruble ($3,500) bill using Ponomaryov's phone.
On Monday, the Investigative Committee said in a statement that the State Duma confirmed that Ponomaryov had given away his SIM card but that Udaltsov had refused to answer investigators' questions, citing Article 51 of the Constitution, which states that no one is obliged to give incriminating evidence about themselves.
Udaltsov faces up to 10 years in jail for allegedly orchestrating violence at a May 6 opposition protest on Moscow's Bolotnaya Ploshchad. Fellow Left Front activists Leonid Razvozzhayev and Konstantin Lebedev are suspects in the same case.
The charges against Udaltsov, Lebedev and Razvozzhayev are based on an expose aired on NTV television in October that showed men resembling the trio in talks with senior Georgian politicians about how to mount riots in Russia.