The chief prosecutor of Austria's supreme court sharply criticised on Monday a lower court's 2011 ruling against extraditing the Kazakh president's former son-in-law over the murder of two bankers.
The June 2011 decision by Vienna's district court not to extradite Rakhat Aliyev because he might not receive a fair trial in his home country was taken "without sufficient reasoning," the office of the chief prosecutor said.
The comments were in a letter sent to and made available by a law firm representing the families of the two bankers whose bodies were discovered in Kazakhstan last May, four years after going missing.
Lawyer Gabriel Lansky told AFP that the decision not to extradite Aliyev, already sentenced in absentia to 40 years in prison, cannot be appealed but that he would be taking the case to the European Court of Human Rights.
"This is a major development because the top jurists in the land ... have said clearly that legal mistakes were made," Lansky said.
"This statement makes it clearer that the victims were messed around for years. It was because of the extradition procedure that the case was not investigated."
Aliyev, the ex-husband of President Nursultan Nazarbayev's eldest daughter Dariga and a former ambassador to Austria who has said he is the victim of a political witch-hunt, is now in Malta, Lansky said.