A Norwegian musician with links to mass killer Anders Behring Breivik has been arrested in France on "suspicion he was preparing a major terrorist act".
The French interior ministry said Kristian "Varg" Vikernes constituted "a potential threat to society".
He was arrested in central France after his wife bought four rifles.
Vikernes, described by French officials as a neo-Nazi, had in the past received a copy of a manifesto from Breivik, who killed 77 people in Norway in 2011.
Breivik planted a bomb in central Oslo and went on a shooting spree on the nearby island of Utoeya in July 2011. He was imprisoned for the maximum 21-year term last year.
An official at the Paris Prosecutor Office said Breivik sent a copy of a manifesto setting out his ideology to Vikernes, who is also a convicted murderer.
The official said: "There were several indications that made the services fear he could possibly carry out a violent act."
Kristian Vikernes was arrested along with his French wife in Correze, a region in central France.
Police sources said officers were searching their home for weapons and explosives.
Officials say Vikernes' wife had a legal firearms permit when she bought the four rifles.
Vikernes, a black-metal musician and writer known as Varg, was convicted in 1994 of stabbing a man to death in Oslo and burning down several churches.
He was released in 2009 and moved to France with his wife and three children.
On his website, Vikernes discusses Breivik's manifesto, but also criticises him for killing innocent Norwegians.
Breivik's 1,500-page manifesto outlined his planned crusade against Muslims, who he said "were taking over Europe and could only be defeated through a violent civil war".
Kristian Vikernes founded the one-man music project called Burzum in the early 1990s, which soon became a fixture of Norway's black metal scene.
The word "burzum" means "darkness" in the Black Speech, a fictional language crafted by JRR Tolkien.
"In 1991, 18-year-old guitarist Varg Vikernes founded one of Norwegian black metal's most important bands upon a simple, yet powerful, platform: to bring darkness into the world," the magazine Guitar World wrote in 2010 after interviewing him.
But he soon became involved in violent crime. In 1992, he took part in the arson of at least three churches in Norway.
The following year, he stabbed to death fellow Norwegian guitarist Oeystein "Euronymous" Aarseth, who played with him in the black metal band Mayhem.
Vikernes was given Norway's maximum 21-year sentence and released from prison in 2009 after serving 16 years.
Since then he has continued releasing music and writing.
Through his writings he promotes what he calls "Odalism", an ideology based on the idea that White Europeans should re-adopt "native European values".
It includes racism, anti-Semitism and elements of ethnic European paganism.