The Central Bank first deputy chairman, Andrei Kozlov, 41, was killed in Moscow September 13 last year. He was in charge of money laundering issues and closed down Frenkel's VIP-Bank three months before the killing.
"We have appealed with the Basmanny Court against the investigator's decision to charge Frenkel as the main suspect in the murder of Kozlov," lawyer Igor Trunov said.
"The decision does not mention a time, place or how the murder was committed," Trunov said. "It is unclear what Frenkel is being accused of, and therefore we request the court to declare the case file baseless and free Frenkel from custody."
The Basmanny Court of Moscow will consider the appeal Wednesday, February 28.
Frenkel, 35, was arrested January 15. His lawyers appealed the decision but the Moscow City Court quashed the appeal February 12.
While in custody, Frenkel published two letters, accusing the Central Bank of accepting bribes for allowing banks to join the deposit insurance system. Prosecutors are investigating his accusations.
Police arrested another suspect in the case, Liana Askerova shortly after Kozlov's murder. She gave testimony implicating Frenkel. His defense said Askerova's words were the only evidence prosecutors had against their client.
In all, seven people have so far been detained in connection with the murder of Kozlov, widely described in the media as an honest official who led a crusade against money laundering and other criminal banking practices. Some of them have reportedly confessed to having been hired to kill the official.