Rakhat Aliyev was found Tuesday morning in the bathroom of his cell in Vienna's Josefstadt jail, prison director Peter Prechtl told the Austria Press Agency.
Aliyev, an estranged former son-in-law of Kazakh President Nursultan Nazarbayev, was charged in December over the kidnapping and killing of two bank managers in Kazakhstan. His trial had been expected to start in March or April. Aliyev had said he was innocent.
Aliyev, 52, was dismissed as Kazakh ambassador to Austria in 2007 after accusing Nazarbayev of totalitarianism. He was sentenced in absentia to 40 years in jail in 2008 on charges including attempting to overthrow the government.
Austria rejected requests for his extradition to Kazakhstan but opened its own investigation.
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Klaus Ainedter, a lawyer for Aliyev, said he had visited him on Monday and "there could be no talk at all of a danger of suicide."
"We trust that his death will be investigated very thoroughly and that the cause of death will be established clearly," he told APA.