Such caveats aside, a prevailing theory among lab-leak proponents has been that SARS-CoV-2 was not simply brought into the Wuhan lab, but was somehow engineered there, given that many of its scientists routinely perform genetic research on coronaviruses and may also have “collaborated on publications and secret projects with China’s military,” according to a U.S. State Department fact sheet released during the last week of the Trump administration. On March 9, a Washington Post columnist, citing an unnamed State Department official, suggested that the Biden administration — while stopping well short of endorsing any particular theory regarding the origin of the virus — did not dispute many of the points made in that fact sheet.

Still, skeptics who doubt the lab leak hypothesis say SARS-CoV-2 doesn’t look anything like an engineered virus. Instead of appearing in discrete chunks, as would be expected with a genetically-engineered microbe, the differences with RaTg13 are distributed randomly throughout the viral genome. In an email to Undark, University of Chicago emeritus virology professor Bernard Roizman wrote that “we are many, many years away from a complete understanding of viral gene functions and regulation — the key elements critical for construction of lethal viruses.”

The virus does have an inexplicable feature: a so-called “furin cleavage site” in the spike protein that helps SARS-CoV-2 pry its way into human cells. While such sites are present in some coronaviruses, they haven’t been found in any of SARS-CoV-2’s closest known relatives. “We don’t know where the furin site came from,” says Susan Weiss, a microbiologist who co-directs the Penn Center for Research on Coronaviruses and Other Emerging Pathogens at the University of Pennsylvania’s Perelman School of Medicine “It’s a mystery.” Although Weiss says SARS-CoV-2 is unlikely to have been engineered, she adds that the possibility that it escaped from a lab can’t be ruled out.