* Historian uncovered Stalin-era mass graves
* Acquitted of child pornography charges in April
* New sex offence accusations have been made
* Members of liberal elite say Yuri Dmitriev is being framed (Adds court ordering two-month detention for Dmitriev)
By Andrew Osborn
MOSCOW, June 28 (Reuters) - A Russian historian whose exposure of Soviet leader Josef Stalin's crimes angered officials was accused on Thursday by state investigators of sexually assaulting his adopted daughter, after having been cleared of similar charges in April.
The new accusation was made as Russia hosts the soccer World Cup, an event opposition politicians have accused the authorities of using to try to bury bad news at a time when Human Rights Watch says the country is experiencing its "worst human rights crisis" since the Soviet era.
Some of Russia's leading cultural figures have said that Yuri Dmitriev is being persecuted because his focus on Stalin's crimes jars with the Kremlin narrative that Russia must not be ashamed of its past.
His real crime, they say, has been dedicating himself to documenting Stalin's 1937-38 Great Terror, in which nearly 700,000 people were executed, according to conservative official estimates. Dmitriev, 62, found a mass grave with up to 9,000 bodies dating from the period.
A European Union spokeswoman on Wednesday said the bloc regarded the case against Dmitriev, which it said had already obliged him to spend 13 months in custody, as "dubious" and expected Russia to drop it.
On Thursday, investigators set out new charges against him. Interfax cited Dmitriev's lawyer, Viktor Anufriev, as saying a court in the northwestern city of Petrozavodsk ordered Dmitriev to be detained for two months.
A court in northwest Russia cleared Dmitriev of child pornography charges involving his adopted daughter in April after a long campaign by human rights activists to free him.
However, a higher court in the Karelia region overturned his acquittal on June 14 on the basis of an appeal by state prosecutors and Dmitriev was detained by police again on Wednesday evening.
On Thursday, state investigators said they had opened a new criminal case against him, this time alleging sexual assault, a crime that carries a prison sentence of up to 20 years.
Investigators said in a statement they would ask a court to remand him in custody later on Thursday while they investigated his alleged crimes which they said happened between 2012 and 2016.
Anufriev told Reuters earlier on Thursday that his client denied the new accusations, which he called "invented," and suggested that investigators had opened the new case to try to show they had not been wrong the first time round.
The Investigative Committee of Karelia, whose investigators submitted the original case for prosecution, have previously not responded to Reuters' questions about whether there was a political side to the case, saying only that they are guided by evidence that they collect.
Dmitry Gudkov, an opposition politician who hopes to run in Moscow's mayoral race later this year, said he thought Dmitriev was being persecuted for his work as a historian.
"Who is so bothered about a person who has committed only one 'offence' -- to investigate the crimes of the Stalin era?" Gudkov asked on Thursday on social media.
"Russia, the legal successor of the USSR." (Editing by Gabrielle Tétrault-Farber, William Maclean)
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