The change comes as an intelligence official says there is no signals or human intelligence backing up the idea, while lawmakers press the administration to turn over any evidence.
The U.S. intelligence community is investigating whether or not the virus originated in a lab, but it "concurs with the wide scientific consensus that the COVID-19 virus was not manmade or genetically modified," the Office of the Director of National Intelligence said in a statement last week.
While Pompeo has said he doesn't doubt the intelligence community assessment, he has boosted the unproven theory the first human infection came from an accidental or intentional release at the Wuhan Institute of Virology. He first told ABC News' "This Week" Sunday that there was "enormous evidence" supporting that unproven theory, before shifting slightly Wednesday to say there's "significant" evidence, but the U.S. doesn't have "certainty" yet.
But in interviews Thursday, Pompeo shifted again, telling a conservative talk radio host, "There's evidence that it came from somewhere in the vicinity of the lab, but that could be wrong."
"We've seen evidence that it came from the lab. That may not be the case," he said in a second talk radio interview.
Republican Sen. Tom Cotton, a China hawk from Arkansas who has also boosted the lab theory, told Fox News this week that the evidence is "circumstantial," but "points directly to the labs." Asked about what kind of evidence the U.S. government has, Pompeo told CNBC Thursday, "One man's direct is another man's circumstantial."
U.S. intelligence officials have been more careful, including the ODNI statement, which made clear the virus's natural origins.
An intelligence official briefed on the situation told ABC News that there is so far no signals or human intelligence backing up the speculation that the lab was the culprit. It also doesn't appear any person or neighborhood connected to the lab became sick at the start of the outbreak, the official said.
"Sometimes political figures use the general term 'intelligence' to include raw reports that are not finished, analyzed intelligence. Raw intelligence is rarely conclusive on a specific topic. Picking one raw report to support a position can be misleading," said Mick Mulroy, a former senior Pentagon official under President Donald Trump and CIA paramilitary operations officer. "The intelligence community would not rely on one report for an assessment on an issue as complex as this. Their assessment would include intelligence from multiple sources and be peer-reviewed."
Reports from the closest U.S. allies have also cast doubt on Pompeo's statement. Known as the "Five Eyes," the U.S., United Kingdom, Australia, Canada and New Zealand cooperate and share signals intelligence, which intercepts signals like communications or radar.
"There's nothing that we have that would indicate that was the likely source," Australia's conservative Prime Minister Scott Morrison said of the Wuhan lab last Friday. Days later, the Australian newspaper The Sydney Morning Herald reported that the theory around the lab's role is "mostly based on news reports and contained no material from intelligence gathering," citing Australian intelligence officials.
"The fact that Australia came out so strongly that they do not believe the line that it all originated in a lab is significant," said Mulroy, now an ABC News contributor, because they're part of the "Five Eyes," "generally see all our intel" and are "well-known for the capabilities in China."
Members of Congress have pressed the Trump administration to provide evidence showing the lab's responsibility. Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, said in a letter Thursday he'd been requesting a briefing by the department for a month now on "what intelligence, if any, the U.S. government has regarding the origins of the virus."
The top Senate Republican, Mitch McConnell, wouldn't back Pompeo's statement either, telling reporters Tuesday, "I don't think we know, except we know it was in China."
Instead of providing details of the "enormous evidence," Pompeo pivoted questions Thursday to demand transparency from China, saying the Chinese government's lack of transparency has halted any efforts to find out how that first transmission took place.
China still has not provided samples of the virus to other countries, instead having its scientists share the virus's genome online in January. It also shut down and sterilized the wet market in Wuhan, where live and freshly killed animals are sold and the virus might have originated, one month before World Health Organization scientists, including two Americans, were given access to the country. While some of those scientists traveled to Wuhan, neither American did.
Critics say brow-beating the Chinese government is not an effective way to get them to open up and provide WHO or other outsiders access to the Wuhan lab, the market or other possible sites. But Pompeo dismissed that again Thursday, telling one host, "We're not raising the rhetoric. We're simply trying to protect the world from a global pandemic by sharing what we know."
He went further on The Steve Gruber Show, setting up U.S.-Chinese relations as a generational challenge between different values systems and agreed with Gruber that China is "the most dangerous adversary for the United States and for all Western governments."
"The whole world can now see that this regime, this authoritarian regime, is just different than we are," said the top U.S. diplomat. "We can see the challenge this presents to our kids and grandkids, and we're determined. We're going to do the right things by building up our military. We're going to do the right things diplomatically."
"In the end, the Chinese Communist Party will have to decide: Do they really want to participate as a member of civilized society, the nations that work toward better outcomes for people all across the world, or are they going to do what we've seen?" he added.
This isn't the first time Pompeo has cast relations with China as a sort of clash of civilizations. He gave a major address last October when he said the Chinese Communist Party is "truly hostile to the United States and our values."
But he has escalated that argument and started to hammer it more often, especially in interviews with conservative media -- 28 of which he's done in the last three weeks -- and near weekly press conferences since March.