A former Russian paramilitary commander who claimed asylum in Norway earlier this month has been arrested by police.
Andrey Medvedev is being held under the Immigration Act, police spokesperson Jon Andreas Johansen told the BBC.
His lawyer, Brynjulf Risnes, told the BBC that the 26-year-old had been moved to a detention centre in the Oslo area.
Mr Medvedev, who crossed into Norway from Russia's far north two weeks ago, is believed to be the first member of the Wagner Group to defect to the West.
The mercenary group - which is believed to have close ties to the Kremlin - has been used in many Russian operations. UK officials estimate it makes up 10% of Moscow's forces in Ukraine.
The move came after "police concluded his situation was very dangerous," Mr Risnes told the BBC. "This is what everyone wanted to avoid, but we are looking for solutions."
Previously, he had been staying at a safe house.
Mr Medvedev's arrest meant he would be under increased security, the lawyer added. But he emphasised that the former mercenary was still being treated as a witness.
Mr Medvedev claims to have witnessed a host of war crimes - including seeing "deserters being executed" by the Wagner Group's internal security service - while fighting in Ukraine's eastern Donbas region.
Vladimir Osechkin, founder of the exiled Russian Gulagu.net rights group, said the ex-commander decided to leave after being informed that his contract would be extended indefinitely, and witnessing the group's "terroristic methods".
Mr Medvedev has indicated he is willing to testify against senior figures in the mercenary group.
In a statement seen by NRK, a special Norwegian police unit which interviewed Mr Medvedev said it was interested in his history in the group, which is believed to be heavily involved in two of Ukraine's bloodiest recent battles - in Soledar and Bakhmut.
After news of Mr Medvedev's arrest broke on Monday, Gulagu.net wrote on social media that the ex-commander had been told he would be deported from the country.
He feared being "brutally murdered" if returned to Russia, the founder of the rights group added. But the Norwegian authorities have not suggested Mr Medvedev would be deported.
Mr Risnes told the BBC that it was "absolutely not true" that his client was facing deportation, but said there may have been a misunderstanding between Mr Medvedev and Norwegian police.
Mr Osechkin did not immediately reply to a BBC request for comment, instead directing reporters to the group's statement on social media.
"We are not looking to whitewash Medvedev. He has done many bad things in his life," Mr Osechkin wrote on Facebook.
"But he has seen the light and is willing to cooperate with Norwegian and international authorities regarding the Wagner Group and its founder Yevgeny Prigozhin," he added.