The exchange was facilitated by Russian peacekeepers who have been deployed to Nagorno-Karabakh for five years under the peace deal.
Yuri Dmitriev's real crime, according to activists, was dedicating himself to documenting Stalin's 1937-38 Great Terror and discovering a mass grave with up to 9,000 bodies dating from the period.
China seeks to increase ties in the ex-Soviet Central Asia region which Russia has traditionally viewed as its backyard.
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