Deeper investigation is needed to find out where SARS-CoV-2 came from, according to a letter signed by 18 scientists from various institutions in North America and Europe. Published today (May 14) in Science, the letter notes that the available evidence about the virus’ origins doesn’t allow researchers to rule out either the hypothesis that the virus spilled over from animals, or the idea that it was accidentally released from a laboratory.

“We must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data,” the authors write. “A proper investigation should be transparent, objective, data-driven, inclusive of broad expertise, subject to independent oversight, and responsibly managed to minimize the impact of conflicts of interest.”

Officials at the World Health Organization (WHO) have previously called the theory that SARS-CoV-2 escaped from a lab in Wuhan “extremely unlikely.” Instead, investigators from the organization who spent nearly a month in Wuhan researching the outbreak’s origins favored the hypothesis that the virus originated in an animal species and spilled over into humans, perhaps via an intermediate host. A number of virologists have concurred with this view.

See “WHO Discounts Idea that SARS-CoV-2 Leaked from a Lab

However, in a statement in March, WHO Director General Tedros Adhanom Ghebreyesus noted that, although it was still unlikely that the novel coronavirus had come from a lab leak, more research was needed to conclusively rule the theory out, according to BBC News.

The WHO-convened investigation has been criticized by the US and other countries. A March 30 statement on behalf of the governments of the US, UK, Australia, and a number of other countries described “shared concerns that the international expert study on the source of the SARS-CoV-2 virus was significantly delayed and lacked access to complete, original data and samples. Scientific missions like these should be able to do their work under conditions that produce independent and objective recommendations and findings.”

Citing this statement, the authors of the Science letter say that “greater clarity” is needed on the origins of the pandemic. They call for public health agencies and research laboratories to make their records public, and for investigators’ analyses to be replicated by “independent experts.”

See “Theory that Coronavirus Escaped from a Lab Lacks Evidence

Responding to the publication of the letter, virologist Angela Rasmussen of the University of Saskatchewan’s Vaccine and Infectious Disease Organization tells The New York Times that she supports additional research on COVID-19’s origins, but adds that the lab-leak theory has become politicized and does not represent the most likely scenario. “There is more evidence (both genomic and historical precedent) that this was the result of zoonotic emergence rather than a laboratory accident,” she tells the Times.