Trinh Xuan Thanh, 51, disappeared in July last year after he was accused of mismanagement at a subsidiary of national oil and gas giant PetroVietnam, resulting in losses of some USD 150 million. Vietnamese police issued an arrest warrant in September.
This week, Vietnamese authorities said he turned himself in to police in his homeland yesterday.
German authorities, however, believe that he was kidnapped in Berlin on July 23. They say that he had sought asylum in Germany an application that hadn't yet been processed and that Vietnamese authorities had sought his extradition.
The kidnapping, he added, "is an unprecedented and flagrant violation of German and international law" and "has the potential to negatively influence relations in a massive way."
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Vietnam's ambassador was summoned to the foreign ministry yesterday and was told that Germany demanded that Thanh be returned so that the asylum and extradition proceedings could be conducted properly.
Schaefer said that German and Vietnamese officials had met on the sidelines of the Group of 20 summit on July 7-8 to discuss Hanoi's wish to have Thanh extradited.
"We reserve the right to draw further consequences if necessary at a political, economic and development policy level," he added.
Berlin prosecutors said they couldn't comment on the case beyond confirming that there is an investigation. Calls to the Vietnamese embassy in Berlin were not answered. Thanh was chairman of PetroVietnam Construction Joint Stock Corporation until 2013 when he was appointed to several senior government positions, including vice chairman of Hau Giang province in the southern Mekong Delta.