Russia’s most well-known opposition politician unexpectedly died Feb. 16 in an Arctic penal colony, and his family have been fighting for more than a week to have his body returned to them.

Russian authorities have said the cause of the Russian dissident’s death is still unknown and refused to release his body for 2 weeks as the preliminary inquest continues.

National Security Council spokesman John Kirby said the U.S. had not determined how Navalny had died, but insisted that the ultimate responsibility lies with Russian President Vladimir Putin.