Pyongyang had claimed Warmbier contracted botulism and fell into a coma after being imprisoned, but doctors at the University of Cincinnati Medical Center have said they found no traces of active botulism.
On Tuesday, President Donald Trump called North Korea's treatment of Warmbier a "disgrace," saying Warmbier's death could have been avoided if he was brought home earlier.
The 22-year-old's death personally affected Trump, who expressed personal outrage and told aides about the "inhumanity" of North Korea's behavior, a person with direct knowledge told NBC News.
The president was moved in a similar way when he saw images of Syrian children after a chemical attack, the source told NBC. After the images surfaced, Trump called the chemical attack "an affront to humanity" and subsequently approved a missile strike on the Syrian airfield that had launched the chemical weapons.
In a Tuesday tweet, Trump said that while he "greatly" appreciated Chinese efforts to rein in North Korea, "it has not worked out."
It was not immediately clear if Trump's tweet indicated a policy shift toward either North Korea or China, a longtime ally of the isolated state and its largest trading partner.
An op-ed published by state-run Chinese newspaper Global Times pushed back on Trump's tweet, saying "The US always blames China for not doing enough when Washington is at a loss over the North Korean nuclear issue."
Anxieties about North Korea's nuclear capabilities have escalated amid recent missile tests.
The Global Times op-ed asserted that the U.S. has "absurd" expectations for Beijing to solve the issues on the Korean Peninsula and overestimates China's influence on Pyongyang.
"Sino-US exchanges on the issue must be based on reality and consider the interests of both sides," it said.
In an apparent attempt to smooth tensions, Mattis said that China and the U.S. have the same end goals in mind when it comes to North Korea. Last week, he told Congress that the U.S. is "exhausting all possible diplomatic efforts" to avoid what would be a "catastrophic war."
The retired four-star general said while the U.S. would prevail, it would be "a war more serious in terms of human suffering than anything we have seen since 1953."
When a reporter asked about the president's tweet in Wednesday's briefing, Mattis said that he believes Trump's view of North Korea represents that of the American people and their "frustration with a regime that provokes and provokes and provokes and basically plays outside the rules, plays fast and loose with the truth."
— NBC News and CNBC's Jacob Pramuk and Jeff Daniels contributed to this report.