The family of a wealthy Russian businessman wants an independent, international expert to investigate the cause of his death.
Oleg Burlakov, who had a net worth of $650 million, according to Forbes, died on June 21 at the age of 72.
His wife and daughter said in a statement to Forbes that they learned of his death from his lawyers.
They said they had previously requested information about his condition when it was reported he was ill, but never received anything.
Forbes said he may have died in Moscow from the coronavirus.
Burlakov was the target of an assassination attempt in 2018 when unknown assailants opened fire on his car. He was not injured in the attack.
The businessman had been living in Monaco where he was engaged in a bitter divorce with his wife Lyudmila Burlakova.
Burlakov, who was engaged in secret military projects during Soviet times, managed to keep a low profile as he built his enormous fortune in the years following privatization.
Forbes called him one of the most secretive individuals on its list of the wealthiest 200 Russians.
Burlakov sold off his cement company in 2007 for about $1.2 billion, according to Forbes, and his oil business in 2014 for $1 billion.
He plowed about $250 million into a 107 meter-long yacht called Black Pearl, which can be powered by solar energy and wind.
Burlakov still owned a stake in a bank, a paint and varnish factory, as well as a real estate firm.