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ST. PETERSBURG, Russia (AP) — Valery Mitko, a renowned Russian polar scientist, has died while under house arrest on charges of treason. He was 81.

Mitko died Monday in St. Petersburg of an unspecified illness, according to the Pervyi Otdel human rights group that defended him. It said that Mitko was discharged from a hospital just a few days ago and was unable to walk.

Mitko, a widely-acclaimed researcher of the Arctic region and one of the country’s leading hydroacoustics experts, was accused of revealing sensitive data during a 2018 academic trip to China. He had denied the charges. Mitko had remained under house arrest since being detained in February 2020.

After retiring from the Russian navy in 1994 after 30 years in the ranks, Mitko turned to science. He taught at several universities and served as the president of the St. Petersburg-based Arctic Academy, an association of scientists that he founded that specialized in polar research.

In the past, Mitko suffered several heart attacks and had other chronic illnesses. Pervyi Otdel said that Mitko’s condition was exacerbated by stress and house arrest. It noted that Mitko was living with his wife, who has been bedridden.

The group described Mitko as another victim of “the repressive system that persecuted him for a crime he hasn’t committed.”

Mitko’s case was one of a series of spy probes in which Russian scientists were accused of leaking sensitive data. The accusations against them have been criticized as baseless by their defenders and human rights groups.

No funeral arrangements for Mitko were immediately announced.