An 81-year-old Russian scientist placed under house arrest after being charged with high treason two years ago has died.

The Pervy Otdel (First Unit) human rights group said on October 2 that Valery Mitko died after suffering his fourth heart attack. The group noted he had been barred from taking regular walks outside of his apartment in St. Petersburg.

Mitko was placed under house arrest in February 2020 for allegedly transferring classified materials to China during regular visits he made there as a teacher.

Mitko denied the charges. He and his lawyers insisted the only materials he took to China were related to his scientific and teaching work.

Russian media reports said at the time that Mitko was accused of delivering top-secret information to China about hydroacoustic research and the detection of submarines.

For years Russian scientists had been encouraged to cooperate with fellow researchers in other countries, but those contacts forged with their counterparts abroad have recently led many to be arrested on treason charges they deny.

Kremlin critics say the charges often stem from unfounded paranoia and attempts to stifle any dissent.