Members of the public alerted police after becoming concerned about the 19-year-old's behaviour.
Sentencing him on Friday, Judge Mark Lucraft KC said: "When it comes to the minimum term that you will serve, I make it plain that I am not ordering that you are to be released at the end of it.
"In my judgement you are someone where there is a significant risk to members of the public or serious harm," the judge told King.Police believe King turned from being a "troubled young man" to a potential "self-initiated terrorist" while stuck at home during the Covid-19 lockdowns. Officers from the Metropolitan Police arrested King at his home at Wickford in Essex on 18 May last year.
He had changed his WhatsApp status to "kill non-Muslims" and recorded himself rapping to his own lyrics about fellow terrorists in Belmarsh Prison and detonating a bomb, officers found. Several of the mosques he attended warned him several times about his behaviour, and one decided he was no longer welcome, Mr Jarvis told the court.
Evidence of King's efforts to buy a hunting knife online helped police build the case against him - he had to give passport details to prove his age, under new rules brought in since 2019.
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