A panel of scientists fiercely debated Tuesday whether COVID-19 stemmed from a laboratory accident or naturally spread from animals to humans, with one expert declaring there was “zero” evidence for a natural origin of the pandemic that killed millions around the world.

Rutgers University molecular biologist Dr. Richard Ebright said in his opening statement before the Senate Homeland Security Committee that the “large preponderance of evidence indicates SARS-CoV-2, the virus that causes COVID, entered humans through a research incident.”

Ebright, who was joined in arguing for the so-called “lab-leak theory” by Dr. Steven Quay, a former professor at Stanford University School of Medicine, added that “no — zero — secure evidence points to COVID’s natural origins.”