Russian prosecutors have reopened a criminal investigation into imprisoned lawyer Sergei Magnitsky, who died while in detention after claiming to have uncovered a multimillion-dollar tax fraud.

Russia's General Prosecutor's office reopened the case Tuesday. No reason was given for the decision.

Magnitsky, who worked for a U.S.-owned investment fund in Russia, accused Russian police officials of stealing fund documents as part of a scheme to pocket hundreds of millions of dollars. Those same police officials arrested Magnitsky in 2009 on what his supporters say were phony tax evasion charges.

He spent nearly one year in jail in poor health, denied the care that his supporters say could have saved his life.

Last week, the U.S. State Department imposed a travel ban on dozens of Russian officials suspected of involvement in Magnitsky's death. U.S. law requires the State Department to deny visas to anyone accused of human rights violations, including torture.

Russia's Foreign Ministry responded harshly to the visa restrictions, saying it would not leave such unfriendly steps unanswered. The ministry promised to take adequate measures to protect the rights of Russian citizens against what it called “unjustified moves” by foreign states.