Police are searching the home of a Norwegian neo-Nazi for weapons and explosives, a source told Reuters.
Kristian Vikernes was arrested in France, where he now lives, after his wife bought four rifles.
"That was at the origin of the investigation ... There were several suspicions that made the services fear he could possibly carry out a violent act," a police official confirmed.
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A man has died following a triple stabbing in London - the fifth killing within six days.
The Earl and Countess of Wessex are marking the occasion privately.
Crossley, who has paranoid schizophrenia, was chased and detained by members of the public.