Meanwhile, the neo-Nazis from the group, found to be an offshoot of the American Atomwaffen Division (the German for "atomic weapon"), which encourages terrorism and worships Adolf Hitler and Norwegian mass murderer Anders Breivik, used pseudonyms to hide their identity as they openly called for violence, promoted cruelty, racism, and misogyny.
The 18-year-old, from Portsmouth, Hants, was arrested on suspicion of encouraging terrorism and the 17-year-old, from London, was arrested on suspicion of supporting terrorism, spreading the terrorist publications, and conspiring to inspire racial or religious hatred.
There have been many alarming messages over several months, including several sent by senior members of the Atomwaffen Division, the American faction of the neo-Nazi organization linked to five murders.
An image posted in the exchange depicted a gun to Prince Harry's head, a swastika, and a caption reading: "See ya later, race traitor!".
Another image shows a woman hanging from a noose and states that white women who date non-white men should be killed. There is evidence to suggest that the group's leader is student Andrew Dymock, 21, according to the BBC.
Mr Koczorowski posted images of himself wearing an Atomwaffen top near Parliament and a video of a British flag being burnt.
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It's believed there are 10 to 15 members of the group in the United Kingdom and some European countries.
The parents of American teenager Blaze Bernstein, allegedly murdered by an Atomwaffen member in what is being treated as a hate crime, told the BBC that people in the United Kingdom should be wary of spin-offs of the group.
Earlier, the group had allegedly killed an adult, named Blaze Bernstein, as he was Jewish and gay.
Two suspects were being questioned in London police stations, the other in Bath. Dymock denies any involvement.
Sonnenkrieg are thought to have no more than 15 members, although Koczorowski was reported to be a pre-ban member of the far larger British fascist group National Action.
Counterterrorism police are investigating threats made against Prince Harry's life after a BBC investigation obtained hundreds of messages sent by extremists.