Abu Dhabi prosecutors have detained a member of their own ruling family after a video was circulated apparently showing him torture another man, BBC reported.
Public prosecutors say they also have begun criminal inquiries into Issa bin Zayed al-Nahyan's actions on the tape.
The investigation is the first reported of a ruling family member in Abu Dhabi, capital of the United Arab Emirates.
Previously, Emirati officials had named Sheikh Issa as the man in the video but said the case was "resolved privately".
"The Public Prosecution Office has officially launched a criminal investigation into the events depicted on video and detained Sheikh Issa bin Zayed al-Nahyan pending the outcome of this investigation," the official news agency WAM reported on Monday.
"This is to ensure that all human rights obligations are met and enforced, that all national laws are applied equally and with transparency to all," WAM said, quoting a statement by the Judicial Department.
On 30 April, after initially dismissing accusations about the video, the Abu Dhabi authorities condemned the torture depicted in the tape and called an inquiry into the case.
The latest statement, issued on Monday, did not give reasons behind the change in approach, or the circumstances of the sheikh's detention.
The advocacy organisation Human Rights Watch described reports of the sheikh's detention as reassuring, but called on the Emirates government to demonstrate complete transparency in the case.
"Secretive prosecutions will not deter further abuses," said Human Rights Watch Middle East director Sarah Leah Whitson.
The video shows Sheikh Issa repeatedly beating the man, Mohammed Shah Poor, a grain merchant of Afghan origin, and running him over with a car. Several other men assist in the tape, including one in a uniform of the security forces.
The incident, said to have taken place in 2004, came to light after US television network ABC broadcast clips of the tape, which was smuggled out of the UAE by a former business associate of Issa bin Zayed al-Nahyan.
Reports say the merchant had lost a consignment of grain belonging to Sheikh Issa worth $5,000 (£3,300). He survived the abuse, but needed extensive hospital treatment.
In a letter to ABC News, the UAE authorities initially said a police investigation that cleared the sheikh had "correctly followed... all rules, policies and procedures" set out by the Police department.
The UAE is a federation of seven wealthy emirates with substantial oil reserves and a large expatriate population. Each emirate is run by a ruling family and citizens are granted few political rights.
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