Russian media on 10 July quoted sources in the Prosecutor-General's Office as saying that Kasyanov is under investigation on charges of gaining property by deception or breach of faith. According to the reports, Kasyanov could face up to five years in prison.
The Prosecutor-General's Office, however, has not officially confirmed the information. A spokeswoman at the Prosecutor-General's Office told RFE/RL on 11 July it had "no official information" about the case. Kasyanov denied the charges in a written statement the same day.
The probe was reportedly initiated by Aleksandr Khinshtein, a journalist and pro-Kremlin deputy in the State Duma, or lower house of parliament. Khinshtein made the first public accusations last week in the newspaper "Moskovskii komsomolets," and also confirmed on 11 July that prosecutors have opened a criminal case against Kasyanov.
Khinshtein was interviewed in a seven-minute television report broadcast by state-owned Channel One on 10 July. Leaning on the handrail of a boat moored across from Kasyanov's dacha, Khinshtein explained that the former prime minister had staged a bogus transaction to buy the residence, estimated to be worth some $30 million, at a bargain-basement price.
The elite dacha, situated on the banks of the river in western Moscow, is surrounded by an 11-hectare park with a tennis court and a private beach.
Khinshtein told RFE/RL's Russian Service today that he was happy prosecutors had launched the investigation against Kasyanov. "I am very happy that they opened [the criminal case]," Khinshtein said. "My prognosis is simple: if the Prosecutor-General's Office is impartial and objective, then the case will be taken to court and Kasyanov will get the punishment he deserves."
Kasyanov served as finance minister under Boris Yeltsin and as prime minister under Putin between 2000 and 2004. He was fired by Putin shortly before the 2004 presidential elections.
He made a comeback on the political scene earlier this year as one of Putin's most vocal critics, describing his policy as misguided and accusing him of human rights violations.
Kasyanov is likely to also have riled the Kremlin by hinting that he might run in the next presidential elections in 2008. This has led to widespread speculation that Kasyanov's possible presidential ambitions could be the actual reason behind the criminal investigation.
Khinshtein rejects such claims. "Kasyanov is the most corrupt prime minister in Russia's recent history," he said. "The tendency of the Russian opposition to let rabble enter its ranks is simply repellent."
Political analysts, however, have little doubt that Kasyanov's political aspirations are connected to the current investigation. Yevgenii Volk, the director of the Heritage Foundation in Moscow, calls the investigation a "political action" against Kasyanov.
"I see Khinshtein's article and the fact that the Prosecutor-General's Office reacted very quickly as an obvious political action aimed at scaring Kasyanov, at removing him from the presidential election race from an early stage, and at preventing him from becoming a serious political figure," Volk told RFE/RL.
The scandal comes just a few weeks after another potential rival to Putin, Yukos founder Mikhail Khodorkovskii, was sentenced to nine years of prison for fraud in a trial largely viewed as politically motivated.