Russian police have detained a university student on suspicion of circulating an Internet video which appeared to show neo-Nazis beheading one non-Slav migrant and shooting another in the head.
Police have not said if the video is authentic, but it has highlighted racist violence in Russia, which is usually directed against dark-skinned migrants from the country's south or former Soviet republics.
The video showed two men with their mouths gagged and hands tied behind their backs, kneeling in front of a swastika flag in a forest. A caption said one was from Russia's mainly Muslim Dagestan region and the second from ex-Soviet Tajikistan.
To a heavy metal backing track, the video appeared to show a masked man hacking off the head of one captive with what looked like a hunting knife.
In another section, a man could be heard shouting "Glory to Russia," then firing a hand gun at the head of the second captive, who was kneeling next to a shallow grave.
"We have detained a young person suspected of uploading this video to the Internet. This man has declared his devotion to national socialist ideas," said an interior ministry spokesman.
"According to preliminary information, he has been distributing this video over the Internet but he is not the author of it ... Experts are still working to establish the authenticity of the video," the spokesman said.
A caption on the video said it was the work of the Rus National-Socialist Party, a previously unheard of group. "Rus" is an historical name for Russia and is used by nationalists.
The student was held by police in the town of Maikop, the capital of the Adygeya region in the south of Russia.
Human Rights campaigner Svetlana Gannushkina said she had seen the video and believed it showed genuine executions.
"It is an absolute nightmare. I have looked through the site they referred to in this video and unfortunately, there were mainly positive comments on it," Ms Gannushkina, who works at the human rights group Memorial, said.
Since the break up of Soviet rule, millions of migrants - many but not all of them Muslims - have traveled to Russia to find work. This has caused tension with ethnic Slavs that frequently spills over into violence.
Many of those convicted of attacking migrants have been young skinheads linked to groups promoting racial purity.
A report last year by rights group Amnesty International said violent racism was out of control in Russia.
Russian President Vladimir Putin has condemned racism, but the Amnesty report said the authorities' response to the racially-motivated attacks had been "grossly inadequate."