RIA Novosti reported Friday that the banking ring called Russian Post had an annual turnover of $500 million.
Authorities did not release the names of six suspects in the case. Only two of the six are expected to be arrested, police said.
A conviction on the charge of illegal banking is punishable by up to seven years in jail.
Russian Post said it was cooperating with the investigation. It was unclear whether or not any Russian Post employees were part of the illegal banking ring, RIA Novosti reported.