Donald Trump first became irate with North Korea when Otto Warmbier, the 22-year-old American student who was detained in North Korea in 2016, died just six days after he was returned to the United States.
He had been arrested for trying to steal a propaganda poster, he returned to the US in a coma in June.
His family declined to confirm his cause of death but said he had been subjected to "awful, torturous mistreatment."
President Donald Trump called North Korea a "brutal regime" after the death was announced.
A general who heads North Korea's rocket command, warned Pyongyang was preparing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around Guam, the tiny US Pacific territory island which hosts 7,000 military personnel on two main bases and has a population of 160,000.t said the plan could be finalised within a week or so and would then go to leader Kim Jong Un for approval.
The US and South Korean military plans to move ahead with large-scale exercises later this month that North Korea, now finalising plans to launch missiles towards Guam, claims are a rehearsal for war.