LONDON (AP) — Britain imposed sanctions Wednesday on six top officials at the Arctic penal colony where Russian opposition leader Alexei Navalny died last week, saying they were responsible for the brutal treatment he suffered in the final months of his life.

The six officials, targeted under U.K. human rights laws, include Col. Vadim Konstantinovich Kalinin, who oversees the prison camp where Navalny was held in solitary confinement for up to two weeks at a time, the Foreign Office said.

The 47-year-old anti-corruption campaigner and former presidential candidate was also denied medical treatment and forced to walk outdoors in temperatures dropping to minus 32 degrees Celsius (minus 26 degrees Fahrenheit), it said.

“It’s clear that the Russian authorities saw Navalny as a threat and they tried repeatedly to silence him,″ Foreign Secretary David Cameron said in a statement.

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The six prison officials will be barred from traveling to Britain, and any assets they may have in the country will be frozen.

Britain also called on Russian authoritie s to release Navalny’s body to his family and urged a “full and transparent” investigation into his death on Friday.

While Navalny’s death is still unexplained, many Western leaders have said they hold Russian President Vladimir Putin responsible.

Navalny returned to Moscow in January 2021 after recuperating in Germany from a nerve agent poisoning that he blamed on the Kremlin. He was immediately arrested and received three prison terms that he rejected as politically motivated.