Lesin was former Russian press minister and a one-time head of the powerful Gazprom-Media Holding group. After his death, his family members were quoted by Russian media as saying that he had a heart attack.
Media `censor`
The chief medical examiner provided no further details. Meanwhile, police spokesman Dustin Sternbeck said the case remained under investigation, the Washington Post reported.
He declined to say whether the post-mortem results meant a crime may have been committed.
Russian foreign ministry spokeswoman Maria Zakharova said Moscow was now expecting "clarification from Washington and relevant official data on the progress of the investigation".
Vladimir Putin spoke of Mikhail Lesin`s (left) "enormous contribution" to Russian media
In 2014, Mississippi Senator Roger Wicker called for an investigation into Lesin, saying his fortune "raises serious questions".
The senator said Lesin had bought property worth $28m (£19.6m) in Los Angeles for his family after finishing work as a civil servant.
Sen Wicker asked how a former civil servant would have been able to buy and maintain expensive property, and expressed concern the purchase may have involved people and groups on a US sanctions list.
The senator also said Lesin "led the Kremlin`s effort to censor Russia`s independent television outlets".
Lesin was for a long time considered one of the most influential figures on the Russian media scene and in the corridors of power.
He worked as an aide to the presidency between 2004 and 2009, when he helped advise on the creation of the news channel Russia Today (now known as RT).
He resigned from Gazprom-Media in 2014. After his death, President Putin spoke of Lesin`s "enormous contribution" to Russian media.
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