Police have "repeatedly refused" to allow the sister of murdered Russian human rights activist Natalya Estemirova access to key case materials, rights organization Memorial said in a report on Thursday.
Memorial's report, presented to President Dmitry Medvedev on July 5, said that Estemirova had investigated a public execution in the Chechen village of Akhinchu-Borzoi a week before she was found shot dead in neighboring Ingushetia on July 15, 2009.
"Over the last year, Russian investigation bodies and the judicial system have repeatedly refused to give the materials of the case to Svetlana Estemirova and her representatives, especially those concerning the Akhinchu-Borzoi execution," the human rights group said in a statement.
Memorial's report said it believed that Estemirova's probe into the execution led directly to her death. Activist Alexander Cherkasov said in 2009 the execution had been carried out in front of the whole village to warn them against "helping militants."
Memorial also said the man suspected of Estemirova's murder - killed in a counterterrorist operation in November 2009 - was not guilty.
Svetlana Estemirova has filed a complaint with the European Court of Human Rights in a bid to force Russian investigators to hand over the materials.
Memorial also said it believed an "effective investigation" was only possible with the active participation of Svetlana Estemirova and her representatives.
It said the participation of the "aggrieved party" was sanctioned by "not only Russia's Criminal Procedural Code, but also the norms of international law."
Natalya Estemirova's murder caused an international and domestic outcry and highlighted the dangers faced by journalists and rights workers in Russia.
Memorial chief Oleg Orlov was cleared last month of slandering Chechen head Ramzan Kadyrov in connection with the case. Orlov had said Kadyrov, a former militant, was "guilty" of the activist's death.