this is "nightline." >> tonight, hunt for the virus. the critical question. growing debate over where covid originated. the dark side of mother nature, or could it have leaked from a lab? and political firestorm. does china have something to hide? as scientists desperately search for answers. new questions about the wuhan lab where the scientist known as bat woman conducted research. >> do you think they were playing with fire? >> absolutely i do. >> "nightline" will be right back. feel like a workout? scrubbing grease and food scrub less with dawn platinum. its superior formula breaks down and removes up to 99% of tough grease and food residue faster. so you can scrub less. dawn platinum is also a go-to grease cleaner for

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martin's family needs it with hassle-free claims, he got paid before his neighbor even got started. because doing right by our members, that's what's right. usaa. what you're made of, we're made for. ♪ usaa ♪ good evening. thank you for joining us. it's one of the most urgent questions in the world today, where did the virus that triggered a global pandemic originate? most scientists still believe it came from nature, moving from animal to humans. but as questions arise about how the chinese handled the initial outbreak and what went on inside a wuhan lab, now some experts consider a lab leak could be possible. here is abc's terry moran. >> you were talking about a

bioengineered monster virus, and you were ruling that possibility out? >> we were ruling that possibility out. >> i definitely have a great respect for nature. but we also have to have a great appreciation for what humans can do. >> no one knows that. >> it's a scientific question that has ignited a political firestorm, now with renewed attention as the possibility of an accidental lab leak gains prominence. president biden has even asked intelligence agencies to redouble their efforts to investigate the origins of covid-19, and multiple probes have been launched around the world, including one led by the u.s. state department and another by the world health organization. >> we need to know the origin of this virus. >> all determined to uncover the truth. could this novel coronavirus come from the dark side of mother nature or from a lab? and if it did come from a lab, was it an accident or could it have been an engineered bioweapon? >> that deadly coronavirus now grows, spreading from wuhan,

china. >> a city of 11 million people, home to several large wildlife markets, potential breeding grounds for viruses. but wuhan is also home to one of the leading laboratories in the world studying bat coronaviruses. and from early on, that had people wondering. >> there is a famous quote in the movie "casablanca." the towns of the world, she all- walks into mine. >> so of all the places in the world where there could be a natural outbreak in the wild, what are the chances that that would not just happen in wuhan collection of bat coronaviruses? >> this is such a political topic. it's so charged. it's a giant puzzle. and like, you know, 95% of the pieces are missing. >> reporter: the evidence we do have is both circumstantial and scientific. the bustling city of wuhan

remote caves in southern china thousands of miles away to the genetic structure of the virus itself. and complicating the task of solving this mystery, the whole topic has become weaponized, a political cudgel. >> they could have stopped it, but they allowed it to come out. >> china virus. >> kung flu. >> with some like david asher who ran the state department's investigation into the origins of the covid virus accusing china of negligence for not alerting the world sooner. >> we're talking lights out. there was no cooperation. and there is still no cooperation. >> reporter: so let's begin with an undisputed scientific fact. viruses are constantly emerging from nature. so when you look at this virus with your expertise, you see the workings of evolution and of nature. you don't see the hand of some diabolical scientist cooking up coronavirus? >> absolutely that just didn't happen at all. >> reporter: robert gary of tulane university is one of the world's leading virologists. >> look at the data a little

closely. i think that most scientists and certainly most virologists have already done this will come to the conclusion that we're looking at a virus that comes from nature. >> reporter: gary argued the genetic code of the virus looks like the kind of thing that happens randomly in nature, not in lab. >> it's really nothing that any experienced virologist or even your worst graduate student would do. >> reporter: and that remains the consensus among most virologists, that this coronavirus has none of the telltale fingerprints of overt bioengineering. so if this had been cooked up in a lab, you'd be able to see that? >> yes. >> and we don't see that? >> no. >> dennis carroll runs the global virome project, a project to identify viruss in the wild that have the potential to trigger future pandemics. carroll and many other virologists say every option should be explored. but there is another likely origin story, wet markets. large open stall markets where live animals are sold.

>> in the wet market, you would find a menagerie of different wild and domestic animals all mixed together, and people will go in and purchase. those conditions are very, very amenable to spillover events. >> reporter: it's fall 2019. >> the house speaker nancy pelosi has decided to open a formal impeachment -- >> reporter: an impeachment inquiry has just been announced. >> hurricane dorian, growing over the past 24 hours -- >> reporter: as a hurricane brews in the atlantic. but something else may have been brewing in wuhan. these satellite photos seen here for the first time show various wuhan hospitals, evidence suggesting the virus was spreading. then in january 2020, all eyes turned to that live animal market in wuhan. but a stubborn puzzling fact emerged. of the first several dozen cases

of covid in wuhan, almost half of them cannot be traced back to that wet market. and the director of the chinese cdc, gao fu later revealed that none of the animal samples taken from the market tested positive for the virus either. adding at the beginning we presumed that the seafood market may have the novel coronavirus, but now it turns out that the market is one of the victims. they had hoped that they would find the host animal at that market, and that's not what they found. what we know now is that the virus was likely spreading in the city before it was detected. the wet market was actually the accelerating agent for the virus and not the initial spark. >> the search for the virus' origin continued. investigators, including some inside the u.s. intelligence community started focusing on a building a few miles from the wet market, the wuhan institute of virology. jamie metzl is a member of the world health organization's expert committee on human genome

editing, and like many others, he has long been concerned about the research that was taking place at the wuhan institute. >> they were doing what some people have called gain of function research, seeing how the world's scariest viruses might infect human cells. >> do you think they were playing with fire? >> absolutely i do. but there were lots of virologists who were in on this. and the idea was let's understand these viruses so we know what we're facing. the counterargument was we're plaing with fire. if it turns out that covid-19 stems from an accidental lab incident from the wuhan institute of virology, it will turn out that that fear was certainly well-founded. >> the wuhan institute is one of china's few level 4 labs with the highest level of biosecurity, enabling them to deal with the world's deadliest pathogens. it's famous in the scientific community, and so is its star

researcher dr. shi zhengli, dubbed the bat woman for her work collecting and studying viruses from bats. >> she said that none of the sequences of the viruses that she collected matches the covid pandemic virus. >> yes. >> doesn't that clear the lab? >> if you believe that one, she has all the information, and two, she is telling the truth. >> dr. shi admitted early in the pandemic that she was worried the virus could have leaked from her lab. >> she told an american science journal that she remembers thinking oh, my gosh, if this is a bat coronavirus, could my lab and the samples i've been working with be involved? when shi zhengli learned that none of the samples that had been collected of this new virus in the earliest cases of coronavirus didn't match any of the samples that she and her team had sequenced from bat caves in years prior, she breathed a sigh of relief. >> the wuhan lab had been

collecting potentially deadly viruses for years, some from an aandoned copper mine in southwestern china. in 2012, there were these six chinese miners who were sent down into a mine in southern china to clean out the batman newer. they all got infected with what now looks like covid covid-19-like systems. three of them died. >> researchers collected viruses where similar coronaviruses have been found. >> one of those coronavirus samples was 96% similar to the virus that has caused this pandemic. politicians and nonscientifics have speculated that it's possible that virus was brought back into a lab and somehow transformed to become our current coronavirus, but what virologists say is that that's simply not the case. >> there is no virus that's close enough that you could use any type of research to get to sars-cov-2. you could hypothesize i suppose

that the investigators at the wuhan institute of virology had some virus that was closer and they just weren't telling us about that. >> do you believe that this virus spent any time in a laboratory? >> before the pandemic, absolutely not. >> when we come back, what happened when bbc reporters tried to go to that cave, and what dr. shi said at an american college about covid-19 leaking from her lab. like many people with moderate to severe ulcerative colitis or crohn's disease, i was there. be right back. but my symptoms were keeping me from where i needed to be. so i talked to my doctor and learned humira is the #1 prescribed biologic for people with uc or crohn's disease. and humira helps people achieve remission that can last, so you can experience few or no symptoms. humira can lower your ability to fight infections. serious and sometimes fatal infections,

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here in the lush mountains

of rural southwestern china in a small mining cave may lie the answers to where the deadly covid-19 virus came from. multiple news organizations, including the bbc attempted to send a team of reporters to this cave, but they say they were turn away at a checkpoint and followed by plainclothes police officers. they were a thousand miles away from wuhan. but questions remain about how much dr. shi, who works at the wuhan institute of virology and those collaborating with her knew, with some suspecting that the virus may have been leaked from the lab. >> with all the very important circumstantial evidence, it became impossible to say that the lab incident origin shouldn't even be examined. >> in january, the state department issued a fact sheet on activity at the wuhan institute of virology that said the lab has collaborated on publications and secret projects with china's military. but dr. shi has denied this.

abc news obtaining this video of shi directly addressing these allegations this past spring at a virtual event held by rutgers university. >> we heard the rumors that it claimed that in our laboratory we have some project blah blah with the army blah blah, but this is not correct. because i am the lab's director and responsible for research activity. i don't know any kind of researcher work performed in this lab. >> david feith was involved in the investigation first launched past fall. you think she is lying? >> it is completely inconsistent with information that the u.s. government had. >> reporter: abc news has also learned from a former state department official that the u.s. government was aware of financial accounts tied to the chinese military that were funding the wuhan institute of

virology and its research. >> the question is was shi zhengli telling the truth when she stated emphatically there was no chinese military involvement prior to the outbreak at the wuhan institute of virology. if there was, her credibility flies out the window. >> that same state department january fact sheet asserted that several researchers at the wuhan institute of virology became sick in the fall of 2019 with symptoms consistent with both covid-19 and common seasonal illnesses. abc news has confirmed from a separate u.s. intelligence report that three researchers from the lab were admitted to the hospital for care in november 2019. the lab has consistently denied these workers were sick with covid, but u.s. officials continue to call on china to release their medical records. robert gary, like many scientists around the world familiar with dr. shi and her

work say the accusations against her and her lab are unfair. >> i see a scientist that's put a lot of her life and her career into looking at these viruses and since back to the original sars. i don't see somebody that's part of a worldwide conspiracy to cover up the origins of sars-cov-2. >> but the chinese government has significantly limited international investigations looking into the origins of the covid pandemic. in january, an international group of scientists working for the world health organization arrived in wuhan. renowned dutch scientist marian koopman was on the trip. how much cooperation did you get from the chinese authorities when you did this investigation? >> there was a lot, but there was also here and there. it was not so easy. it was everything you would wish to see on the table, no. i think it's important to understand just how complex this is.

it takes a while to build a working relationship, and that was part of the process. >> the w.h.o. team released their report this march concluding that the covid-19 virus most likely followed the route through an intermediate animal, calling the laboratory leak extremely unlikely. >> when the report was being announced, dr. tedros, the w.h.o. director general, he immediately said this is insufficient. >> they have said all hypotheses are on the table. >> do you think there are people in the wuhan institute of virology and in the chinese government who know for sure whether or not it's a lab leak? . there are people associated with the wuhan institute of virology, and almost surely elsewhere in the chinese system who know enormous amounts what was going on inside that laboratory in the days before the outbreak when you had sick researcher, and in the months and years before the outbreak when you had research

being conducted of a dangerous variety with viruses similar to covid-19. >> for all the circumstantial evidence and speculation, there is still no proof the virus came from a lab, and only recently have many scientists and others begun to take the lab leak possibility seriously. in february 2020, 27 prominent scientists signed a letter to the lancet, one of the world's leading scientific journals that rejected the lab leak theory. now abc news has reached out to all 27 signers of the letter. a dozen responded to our inquiry. >> many of them are much less definitive about a natural spillover. many are more open to the possibility of a lab leak. we even heard back from one of the experts who had originally signed that letter who now thinks a lab leak is more likely than a natural spillover event. >> reporter: jamie metzl says that original lancet letter made it difficult for him and others to explore anything but the natural origin of the virus. >> everybody got into this group

think, and that was the story. >> people thought you and others raising these questions were coo kooks? >> yes that. >> did. >> now those same questions have gone mainstream, but it could take years to find the answer, if we ever do. do you think we'll ever know for sure where this virus came from? >> we'll know with greater confidence where it came from, yes. >> in the first sars epidemic that. >> did find host animal and intermediate animal. here we are now a year and a half later. does it bother you at all that they have not found, despite searching, any animal that would be helpful to determining the origin of this? >> it doesn't bother me at all. actually finding the reservoir of an emerging virus like that is pretty unusual. it's the exception rather than the rule. it might take years or decades to find that source animal for sars-cov-2. when we come back, a note

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about one of our own. you likely didn't know his name, but you sure knew his spectacular camera work here on abc news for decades. our friend and long-time abc news photographer jim cecil has died after battling lung cancer. he started in the mailroom in 1972, then the assignment desk. as a photographer, he covered so many stories in the u.s. and around the world. he will be missed. our thoughts and prayers are with his family. that's nig"nightline" for this evening. and get your full episodes on hulu. we'll see you right back here same time tomorrow. thanks pour th