Exiled tycoon and former prisoner Mikhail Khodorkovsky has been summoned for questioning in Moscow.
Investigators announced in June that they were reopening a probe into the 1998 killing of a Siberian mayor and considered the Kremlin critic a prime suspect.
Khodorkovsky, 52, denies having any involvement.
He spent 10 years in prison on charges of tax evasion and embezzlement, which have been seen as punishment for challenging President Vladimir Putin’s power.
Since his 2013 release, Khodorkovsky has continued to oppose the Kremlin from exile in Switzerland.