Russian authorities have refused to open a criminal investigation into the death of opposition leader Alexei Navalny who died in an Arctic penal colony, his widow Yulia said Thursday.

Navalny, the fiercest political foe of Russian President Vladimir Putin, died in February in the remote northern prison colony while serving a 19-year sentence on extremism charges that he had condemned as politically motivated. Authorities said he became ill after a walk but have otherwise given no details on Navalny’s death.

Russian officials only said that Navalny died of natural causes shortly after taking a walk and have vehemently denied involvement both in the poisoning and in his death.

In a video published Thursday on social media, Yulia Navalnaya, Navalny's widow, said she received a letter from Russian investigators detailing the cause of her husband's death.

In the video, Navalnaya said she was told Navalny died from a combination of “a dozen different diseases." Investigators told her Navalny finally died from arrhythmia, or an irregular heartbeat.

Navalnaya disputed Russian officials' version of events and said her husband exhibited no instances of heart disease while alive.

The letter, Navalnaya said, was “a lie.”

“They are hiding what really happened that day ... when Alexei became ill, he was not taken to the medical unit, but back to the punishment cell.”

There, she said, Navalny was “dying alone."

The document said officials would not open a criminal investigation into the opposition leader's death because no offense had been committed, Navalnaya said.

“As long as Putin is in power, there will be no investigation,” Navalnaya said, vowing to continue to probe the circumstances around her husband's death.