Valery Pshenichny was arrested in January and accused of embezzling funds.
An investigation was launched in Russia on Thursday into the presumed suicide in prison of an entrepreneur dubbed by the media as the "Russian Elon Musk".
"There are reasons to believe that it is not a suicide," said Larissa von Arev, lawyer for businessman and computer engineer Valery Pshenichny.
The entrepreneur was known for his ambitious and innovative ideas and drew comparisons with the US billionaire boss of Tesla and SpaceX.
Pshenichny was arrested in January and accused of embezzling funds in a contract with the Russian defence ministry. His company, NovIT PRO, had been hired to develop a 3D virtual model of a submarine which could be used by engineers involved in repairing such vessels.
Three weeks later on February 5, he was found hanging in his prison cell in Saint Petersburg. He was 56.
According to his attorney, forensic pathologists discovered "traces of electric burns, fractures and traces of sperm" on the businessman's body.
"Just before his death, his widow Natalia received notes from her husband telling her 'not to pay anyone' and proclaimed his innocence," von Arev said.
Media and NGOs have regularly report cases of extortion and threats within in the jail system.
The Russian penitentary service said in a statement it was opening an inquiry to shed light on the circumstances of the entrepreneur's death.
"This tragic story shows the fact that our rights to life and health are not guaranteed in detention," said the lawyer.
Human rights advocates denounce torture, humiliation and beatings in Russian prisons.