Eighteen scientists from some of the world’s most prestigious research institutions are urging their colleagues to dig deeper into the origins of the coronavirus responsible for the global pandemic.

In a letter published Thursday in the journal Science, they argue that there is not yet enough evidence to rule out the possibility that the SARS- CoV-2 virus escaped from a lab in China, and they call for a “proper investigation” into the matter.

“We believe this question deserves a fair and thorough science-based investigation, and that any subsequent judgment should be made on the data available,” said Dr. David Relman, professor of microbiology and immunology at Stanford University who helped write the letter.

The brief letter was prompted in part by the March 30 publication of a report commissioned by the World Health Organization that sought to discover the origin of the virus that has resulted in the deaths of more than 3.3 million people across the globe.

The authors of that report, which is credited to both the WHO and China, ranked each of four possible scenarios on a scale from “extremely unlikely” to “very likely.”

After considering information, data and samples presented by the Chinese members of the team, the authors concluded the likelihood that the virus jumped from a source animal to an intermediary species and then to humans was “likely to very likely,” while an introduction due to an accidental laboratory leak was deemed “extremely unlikely.”

Other potential pathways the investigators considered were a direct jump from animal to human without an intermediate host (“possible to likely”) and transmission from the surface of frozen food products (“possible”).

But Relman and his co-authors said their colleagues who worked on the WHO investigation did not have access to enough information to draw these conclusions.

“We’re reasonable scientists with expertise in relevant areas,” Relman said, “and we don’t see the data that says this must be of natural origin.”