President Donald Trump faced a bipartisan backlash Thursday after saying he did not believe North Korean leader Kim Jong Un was responsible for the death of American student Otto Warmbier.

"We must remember Otto, and we should never let North Korea off the hook for what they did to him," Sen. Rob Portman, R-Ohio, who represents the Warmbier family's home state, said in a statement a few hours after Trump completed his second summit with Kim in Vietnam.

Trump said Thursday at a press conference in Hanoi at the conclusion of the summit that he did not hold Kim responsible for Warmbier's death.

"I don't believe he knew about it," Trump said of Kim. "He tells me that he didn't know about it, and I will take him at his word."

Trump's comments are a reversal from his past statements. In his 2018 State of the Union address, he criticized North Korea's "depraved character" and blamed the country's "dictatorship" for Warmbier's injuries and eventual death.

Warmbier, 22, was arrested for taking a propaganda banner from a hotel while on a visit to Pyongyang in January 2016 and later sentenced to 15 years of hard labor. He suffered horrific injuries and died shortly after being released from 17 months of detention. The cause of death remains unknown, as does the cause of his injuries.

House Speaker Nancy Pelosi, D-Calif., on Thursday called the president's remarks "strange."

"There’s something wrong with Putin, Kim Jong Un — in my view, thugs — that the president chooses to believe," she said.

Otto Warmbier is taken to North Korea's top court in Pyongyang, North Korea, on March 16, 2016. Kyodo / Reuters file

Sen. Cory Gardner, R-Colo., a member of the Senate Foreign Relations Committee, told Fox News that "the blood of Otto Warmbier is on the hands of Kim Jong Un" and that there is "no doubt in my mind" that Kim knew about Warmbier’s detention and allowed it to happen.

"Look, Otto Warmbier, they made a spectacle out of him to the world. Accusing him of being a traitor, a spy. And they executed him. The blood of Otto Warmbier is on the hands of Kim Jong Un. There is no doubt in my mind that he knew about it, he allowed it to happen, and the responsibility lies directly with Kim Jong Un," he said.

Sen. Mark Warner, D-Va., said Trump's remarks show he believed an "obvious lie" by Kim.

"Otto Warmbier's bogus arrest and brutal murder was an international incident," Warner said. "Of course, Kim knew about it. Apparently, the president of the United States is the only one who believes this obvious lie."

Sen. Ron Wyden, D-Ore., accused Trump of flinching in front of Kim regarding the student's death.

"No American president has ever cowered in front of dictators like @realDonaldTrump," he said in a tweet.

Trump abruptly ended his summit with Kim without any concessions on Pyongyang's nuclear weapons program. Some lawmakers approved of the move, hoping it would keep the door open for more diplomacy and prevent an unsatisfactory deal.

"Walking away from the summit was better than making a bad deal. It’s also the result of a poorly planned strategy," Rep. Adam Schiff, the chairman of the House intelligence committee, tweeted Thursday. "But accepting Kim's denial of involvement in Warmbier’s death? Detestable, and harkens back to Trump's duplicitous acceptances of denials from other dictators."

"Americans know the cruelty that was placed on Otto Warmbier by the North Korean regime," Nikki Haley, Trump's former U.N. ambassador, tweeted Thursday.

Former Republican Sen. Rick Santorum of Pennsylvania told CNN that it was "reprehensible" that Trump gave cover to Kim regarding Warmbier’s death.

"This is the conundrum of Donald Trump for many of us who like his policies and don't like a lot of the things he does and says ... This is reprehensible what he just did," said Santorum, who credited the president for not making a deal with Kim. "I mean he gave cover ... to a leader who knew very well what was going on with Otto Warmbier. And again, I don't understand why the president does this, I am disappointed to say the least that he did it."

When asked about Warmbier's death, Sen. Mitt Romney, R-Utah, a member of the Senate Foreign Relations Committee, told reporters there was "no question" North Korea was responsible for human rights abuses.

“I've never heard someone who did something wrong jump up and say, 'Yeah, I did it, I did it,'” Romney said. "I think there's no question that the government of North Korea has been responsible for a whole host of human rights abuses and atrocities."

Sen. Mazie Hirono, D-Hawaii, said it's hard to believe that Kim didn't know what happened to Warmbier, comparing it to Trump taking Russian President Vladimir Putin's word over U.S. intelligence that Russia didn't interfere in the 2016 elections.

"I think the president is very gullible in this regard," she said. "He seems to have a very odd affinity toward dictators ... it is very odd, and I think it’s misplaced."