Pushed to respond to speculation from a Republican senator, a top White House official said they do not know whether the coronavirus from China could have originated in a laboratory there.

Asked during an Atlantic Council event about comments by Arkansas Sen. Tom Cotton on the possibility that the virus started in a Chinese government lab, national security adviser Robert O’Brien said Tuesday that he had no information about any such theories.

“I’ve seen those reports, and Twitter and the internet are alive with them. I don’t have any information on that one way or the other, so we just don’t know,” O’Brien said. “I can’t comment on that.”

“Sen. Cotton is a ⁠— so I say this about all senators ⁠— very smart, handsome, intelligent senator,” O’Brien said. “He’s been watching this very closely.”

“He’s a smart guy when he comes to these issues, and he’s a leader when it comes to national defense and national security issues,” O’Brien said of Cotton, who is a member of the Intelligence and Armed Services committees.

During a Senate Armed Services Committee hearing at the end of January, Cotton spoke about the possibility that the virus may have been developed by the Chinese government ⁠— which he said could be “worse than Chernobyl.”

Chinese ambassador to the United States Cui Tiankai has criticized Cotton’s suggestion, including in an interview Sunday with CBS News.

“It’s very harmful, it’s very dangerous to stir up suspicion, rumors and spread them among the people. For one thing, this will create panic. Another thing, that it will fan up racial discrimination, xenophobia, all these things, that will really harm our joint efforts to combat the virus,” the ambassador said.

Anne Schuchat, deputy director of the Centers for Disease Control and Prevention, warned about the emergence of rumors in remarks at the National Press Club on Tuesday. She explained current information suggests the virus was transmitted from animal to human, pushing back on the idea that it may have originated from a Chinese lab.

“Based on everything that I know about what is going on with this outbreak and the research that’s being conducted, as well as the genomic sequences that have been posted and the comparison with animal strains, the pattern that we’re seeing is quite consistent with emergence from animal to human acquisition and adaptability or mutations that permit the virus to be easily spread between people,” she said.

Cotton reiterated his concerns in a CNBC interview Tuesday morning.

“We do need to ask questions about the super laboratory that is in Wuhan,” Cotton said. “There’s too many unanswered questions and China has been too deceitful for us to just take the word of the Chinese Communist Party.”

O’Brien said as of Tuesday evening the Chinese government was not allowing medical doctors affiliated with the U.S. government to get access to Wuhan, the city most closely connected with the virus.

“We know where it originated. We don’t know how it originated,” O’Brien said. “We do not have American doctors on the ground. The [World Health Organization], I believe, has a new team in, but that team does not include American doctors.”

“We’ve offered the Chinese the opportunity to have American doctors from [the Centers for Disease Control and Prevention] and [the National Institutes of Health] and other experts come to China and help them,” O’Brien said. “That offer has not been accepted at this point, but that’s an outstanding offer.”