The calls for a response come as spy satellites detected new activity at North Korea's underground nuclear test site for the first time in several weeks, according to two U.S. officials who spoke to CNN.
It jars so strikingly with the fates of most past detained Americans that outside observers are left struggling not only with the mystery of what killed Warmbier but also with what his death means for attempts by Washington and its allies to stop North Korea's pursuit of a nuclear-tipped ICBM that can target the USA mainland.
His family said it was told he fell into a coma soon after his March 2016 sentencing after contracting botulism and taking a sleeping pill.
The coroner's office in Cincinnati, Ohio, launched an investigation into the death of Otto Warmbier, the 22-year-old American student who died Monday just days after being released from a North Korean prison. The president says if Otto Warmbier had been returned home to the USA earlier, "I think the result would have been a lot different".
"We have a great deal of resolve to try to handle the situation and try to hold North Korea responsible for the death of Mr. Warmbier and bring back those three Americans who do remain there", Nauert added.
Last month, a bipartisan group of congressmen, led by Rep. Adam Schiff (D-CA), introduced the North Korea Travel Control Act (H.R.2732) that would prohibit travel to North Korea by Americans without permission from the government.
"Our deepest sympathies are with the family and friends of Mr Warmbier at this time of their tragic loss".
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The U.S. side is expected to press China to cooperate on a further toughening of worldwide sanctions on North Korea.
The agency, founded in 2008 by a British expat, came under fire after Warmbier was flown home in a coma following a flurry of secret diplomatic negotiations between Washington and Pyongyang. He could not speak or move voluntarily and his doctors said he suffered extensive brain damage.
Let's be clear about something: Otto was held prisoner by the North Koreans.
Emails and calls to Young Pioneer Tours' office in the central city of Xian and mobile numbers went unanswered. It says his family objected to an autopsy. A naturalized US citizen who was born in South Korea, he is a businessman in his mid-60s who at one time lived in Fairfax, Virginia. "Surrounded by his loving family, Otto died today at 2:20 p.m.", his mom, Cindy, wrote in a statement.
He was living in China near the border with North Korea and working in a special economic zone in North Korea as the president of a trade and hospitality company.
"It is our sad duty to report that our son, Otto Warmbier, has completed his journey home".
The University of Virginia student died Monday. Al Franken and others when he said in a statement that "the barbaric treatment of Otto Warmbier by the North Korean regime amounts to the murder of a United States citizen".
Warmbier was arrested and imprisoned in North Korea during President Barack Obama's time in office, though North Korea only informed the U.S. of Warmbier's medical condition earlier this month. Warmbier, an American college student who visited North Korea 18 months ago, was sentenced to 15 years to hard labor after the regime convicted him of attempting to steal a propaganda poster from his hotel. Doctors said much of Warmbier's brain matter had been destroyed by his treatment.