Russia wants to question former oil Yukos executive Mikhail Khodorkovsky - once Russia's richest man who was arrested in 2003 and convicted of theft and tax evasion in 2005 - for involvement in the 1998 murder of Vladimir Petukhov.
Petukhov was the mayor of a Siberian town where Yukos had its largest oil production facility. Khodorkovsky denies any involvement, and writes on Twitter the new probe again is politically motivated.
The move by Moscow comes as Khodorkovsky is assuming the role as the most outspoken opposition critic of the Putin regime. He bankrolls a Russian civil society organization called Open Russia that wants to put the country on a more open and democratic path.
Life in exile
Khodorkovsky has lived in exile in Switzerland since his 2013 release from prison, where he spent 10 years on charges of tax evasion, embezzlement and money laundering that were widely seen as punishment for challenging Putin.
An international arbitration court ruled last July that Russia must pay $50 billion for expropriating Yukos' assets. It has not complied and a group of former shareholders has started legal proceedings to have the order enforced.
Russian investigators now say they have new evidence that directly implicates Khodorkovsky in the killing of the mayor and other crimes. The Tass news agency reports Khodorkovsky's spokeswoman Olga Pispanen said there would be no comment.
The Kremlin is suspicious of non-governmental organizations, especially those funded from abroad, seeing them as aimed at undermining Putin's rule.
The Russian government also is fighting a ruling made last year by an arbitration court in The Hague, Netherlands, that it must pay $50 billion to compensate the former shareholders of Yukos, which was bankrupted in the same legal onslaught that sent Khodorkovsky to prison.
A group of former shareholders has initiated legal maneuvers to enforce the order.
Petukhov, who was shot dead, had been leading a public campaign against Yukos over alleged tax arrears. A Yukos security chief was later convicted of his murder.
Vadim Klyuvgant, a lawyer for Khodorkovsky, said the investigators' accusations are "fantasy."
Khodorkovsky himself has suggested this is a response to articles he has written criticizing Putin and to the court settlement compensating Yukos shareholders who lost out when Russia seized the company, split it up and assumed total control.
Information from The Associated Press and Reuters was used in this report.