Britain has issued sanctions against six people who led the penal colony where Russian opposition leader Alexei Navalny died last week, the UK Foreign Office said Wednesday.

The six prison officials, who were sanctioned under UK human rights regulations, include Vadim Konstantinovich Kalinin, who oversaw the brutal prison camp where the anti-corruption campaigner was kept in solitary confinement. Navalny also suffered from being denied medical treatment and from having to walk in temperatures dropping to minus 32 degrees Celsius, the Foreign Office said in a statement.

“It's clear that the Russian authorities saw Navalny as a threat and they tried repeatedly to silence him,'' Foreign Secretary David Cameron said. “That's why we're today sanctioning the most senior prison officials responsible for his custody in the penal colony where he spent his final months.''

He added: “Those responsible for Navalny's brutal treatment should be under no illusion — we will hold them accountable."