Jamie Metzl: We would have to ask the question, "Well, why in Wuhan?" To quote Humphrey Bogart, "Of all the gin joints in all the towns in all the world, why Wuhan?" What Wuhan does have is China's level four virology institute, with probably the world's largest collection of bat viruses, including bat coronaviruses.
Lesley Stahl: I had seen that the World Health Organization team only spent 3 hours at the lab.
Jamie Metzl: While they were there they didn't demand access to the records and samples and key personnel.
That's because of the ground rules China set with the WHO, which has never had the authority to make demands or enforce international protocols.
Jamie Metzl: It was agreed first that China would have veto power over-- over who even got to be on the mission. Secondly--
Lesley Stahl: And WHO agreed to that.
Jamie Metzl: WHO agreed to that. On top of that, the WHO agreed that in most instances China would do the primary investigation. And then just share its findings--
Lesley Stahl: No.
Jamie Metzl: --with these international experts. So these international experts weren't allowed to do their own primary investigation.
Lesley Stahl: Wait. You're saying that China did the investigation and showed the results to the committee and that was it?
Jamie Metzl: Pretty much that--
Lesley Stahl: Whoa.
Jamie Metzl: --was it. Not entirely. But pretty much that was it. Imagine if we have asked the Soviet Union to do a co-investigation of Chernobyl. It doesn't really make sense.
China had ruled out a lab accident long before the WHO team arrived at the airport in Wuhan on January 14 and were greeted by people in full PPE gear.
The team included some of the world's leading experts on how viruses are transmitted from animals to humans. But even though there have been accidental lab leaks of viruses in China in the past that have infected people and killed at least one, no one on the team was trained in how to formally investigate a lab leak.
They were there for a four-week mission, but two of those weeks were spent holed up at this hotel in quarantine. Once out, they had some tense exchanges with their counterparts, a team of chinese experts, over their refusal to provide raw data.
If the virus originated in animals, one of the mysteries has been: how did it travel the thousand miles from the bat caves in southern China to Wuhan? The WHO team thinks it found the answer.
Peter Daszak: What we found as part of this WHO mission to China is that there is a pathway.
Peter Daszak, a member of the WHO team, and an expert on how animal viruses jump to humans, has worked on previous viral outbreaks, including in China.
He says the pathway leads not to the lab in Wuhan but from wildlife farms in southern China directly to the wet market in Wuhan, the Huanan Seafood Market.
Peter Daszak: The theory is that somehow that virus got from a bat into one of these wildlife farms. And then the animals were shipped into the market. And they contaminated people while they were handling them, chopping them up, killing them, whatever you do before you cook an animal.
Lesley Stahl: Wild animals?
Peter Daszak: Yeah, these--
Lesley Stahl: Like what?
Peter Daszak: They're a traditional food. Civets, these are like ferrets. There's also an animal called a ferret badger. Rabbits, which we know can carry the virus. Those animals were coming into the market from farms over 1,000 miles away.
Lesley Stahl: Were you able to test any of the animals found in the Wuhan market for the virus?
Peter Daszak: Well, the China team had done that, and they found a few animals left in freezers. They tested them, they were negative. But the fact that those animals are there is the clue.
Lesley Stahl: But there's no direct evidence that any of those animals were actually infected with the bat virus?
Peter Daszak: Correct. Now what we've gotta do is go to those farms and investigate. Talk to the farmers. Talk to their relatives. Test them. See if there were spikes in virus there first.
Lesley Stahl: So, the team doesn't actually know if any of the farmers or the truckers were ever infected?
Peter Daszak: No one knows yet. No one's been there. No one's asked them. No one's tested them. That's to be done.
Despite those unanswered questions, the WHO team and their Chinese counterparts all agreed that this hypothesis of a pathway - from bat caves to butcher shops like these - is the most likely explanation.
Peter Daszak: Something like 75% of emerging diseases come from animals into people. We've seen it before. We've seen it in China with SARS.
Lesley Stahl: Is the lab leak theory any more or less speculative than the-- your pathway?
Peter Daszak: For an accidental leak that-- that then led to COVID to happen, the virus that causes COVID would need to be in the lab. They never had any evidence of a virus like COVID in the lab.
Lesley Stahl: They never had the COVID-19 virus--
Peter Daszak: Not prior--
Lesley Stahl: --in that lab?
Peter Daszak: --to the outbreak, no. Absolutely. No evidence of that.
Jamie Metzl begs to differ, pointing to the lab's own reports that it sent field researchers to the bat caves who brought back samples with viruses.
Jamie Metzl: We know that among those viruses, one of them is the virus that is genetically most related to the SARS-CoV-2 virus.
Lesley Stahl: But most related isn't the same, right?
Jamie Metzl: Yes, exactly. But we do know that there were 9 viruses at least that were brought back. And it's extremely possible that among these viruses is a virus that's much more closely related to the SARS-CoV-2 virus. And when I put all those pieces together, I said, "Hey, wait a second, this is a real possibility. We need to be exploring it."
Lesley Stahl: The pathway that Peter Daszak and the team have come up with-- now that sounds plausible.
Jamie Metzl: Oh, it's certainly plausible.
Lesley Stahl: Very, seriously plausible.
Jamie Metzl: No, it is plausible. Let's just say that that theory is correct. You would have had an outbreak, perhaps in Southern China where they have those animal farms. You may have seen some kind of evidence of an outbreak along the way.
Lesley Stahl: And there wasn't?
Jamie Metzl: There wasn't.
Lesley Stahl: But listen, your theory is also full of holes.
Jamie Metzl: I wouldn't say it's full of holes, but it's incomplete. That's why we need access to the data in order to prove one hypothesis for another.
Metzl says Peter Daszak has a conflict of interest because of his long-time collaboration with the Wuhan lab.
Peter Daszak: I'm on the WHO team for a reason. And, you know, if you're going to work in China on coronaviruses and try and understand their origins, you should involve the people who know the most about that. And for better or for worse, I do.
He says the team did look into the leak theory during a visit with lab scientists and deemed it 'extremely unlikely'.
Peter Daszak: We met with them. We said, "Do you audit the lab?" And they said, "Annually." "Did it you audit it after the outbreak?" "Yes." "Was anything found?" "No." "Do you test your staff?" "Yes." No one was--
Lesley Stahl: But you're just taking their word for it.
Peter Daszak: Well, what else can we do?
There's a limit to what you can do and we went right up to that limit. We asked them tough questions. They weren't vetted in advance. And the answers they gave, we found to be believable-- correct and convincing.
Lesley Stahl: But weren't the Chinese engaged in a cover-up? They destroyed evidence, they punished scientists who were trying to give evidence on this very question of the origin.
Peter Daszak: Well, that wasn't our task to find out if China had covered up the origin issue.
Lesley Stahl: No, I know. I'm just saying doesn't that make you wonder?
Peter Daszak: We didn't see any evidence of any false reporting or cover-up in the work that we did in China.
Lesley Stahl: Were there Chinese government minders in the room every time you were asking questions?
Peter Daszak: There were Ministry of Foreign Affairs staff in the room throughout our stay. Absolutely. They were there to make sure everything went smoothly from the China side.
Lesley Stahl: Or to make sure they weren't telling you the whole truth and nothing but the truth--
Peter Daszak: You sit in a room with people who are scientists and you know what a scientific statement is and you know what a political statement is. We had no problem distinguishing between the two.
Speaking of political statements...
Geopolitics loomed over the entire inquiry with some tit for tats: Beijing said COVID-19 originated in the U.S.; the Trump administration accused China of a cover-up.
Matt Pottinger: There was a direct order from Beijing to destroy all viral samples -- and they didn't volunteer to share the genetic sequences.
Matt Pottinger, the then deputy national security adviser, quoting from declassified intelligence information, says Beijing also hid that several researchers at the Wuhan lab had come down with COVID-like symptoms and that the Chinese military was working with the lab.
Matt Pottinger: There is a body of research that's been taking place conducted by the Chinese military in collaboration with the Wuhan Institute of Virology, which has not been acknowledged by the Chinese government. We've seen the data. I've personally seen the data.
Lesley Stahl: Why the military? Why were they in that lab?
Matt Pottinger: We don't know. It is a major lead that needs to be pursued by the press, certainly by the World Health Organization. Beijing is simply not interested in allowing us to find the answers to those very pertinent questions.
What the U.S. government does know, he says, is that the Wuhan lab director published studies about manipulating bat coronaviruses in a way that could make them more infectious to humans, and there were reports of lax safety standards at the lab.
Matt Pottinger: They were doing research specifically on coronaviruses that attach to the ACE2 receptors in human lungs just like the COVID-19 virus.
Lesley Stahl: Is that a smoking gun?
Matt Pottinger: No, it's circumstantial evidence. But it's a pretty potent bullet point when you consider that the place where this pandemic emerged was a few kilometers away from the Wuhan Institute of Virology.
The lack of transparency has led to widespread criticism of the WHO for agreeing to China's demands.
Matt Pottinger: The one thing that I wish the WHO had done is to pick up their megaphone and start screaming through it to demand that China be more transparent, that it open its border to allow American CDC officials and other experts from the WHO and around the world to come investigate and to help.
After 15 months and more than 2.7 million deaths worldwide, it was hoped the team would provide some clarity on the origin of COVID-19. But the exercise ends with even more questions than it began with.