Russian investigators say they are considering whether to charge former oil tycoon Mikhail Khodorkovsky with ordering the 1998 killing of a Siberian mayor.

At the time when Nefteyugansk Mayor Vladimir Petukhov was shot dead, he was in conflict with Khodorkovsky's oil company, Yukos, over alleged tax arrears. Khodorkovsky and his associates have denied any involvement and no one has been convicted in the killing.

Tuesday's announcement that investigators were reopening the case and considered Khodorkovsky a prime suspect came as he has become an increasingly strong voice in Russian opposition politics.

Khodorkovsky has lived in exile in Switzerland since his 2013 release from prison, where he spent 10 years on charges seen as punishment for challenging President Vladimir Putin's power. He funds a Russian civil society organization called Open Russia.