A 71-year-old Ukrainian man died in Russia in December awaiting trial for espionage and other crimes, human rights organisation Memorial said on Sunday, citing court documents.
Several Ukrainian citizens have been accused of espionage by Russia following the start of what the Kremlin calls a special military operation almost two years ago.
Memorial said there was little information about Viktor Demchenko, who was thought to have died at a pre-trial detention facility in the southern Rostov region. His case was sent to a military court in the city of Rostov-on-Don in August.
TASS news agency said the court had closed the case against Demchenko, also accused of participation in a "terrorist group" and illegal possession of arms and munitions, due to his death.
It cited law enforcement officials as saying he died on Dec. 31 from consequences of a stroke he had suffered earlier.
It was unclear why information about Demchenko's death had emerged only weeks after.