As the Biden administration intensifies its efforts to track down the source of COVID-19, unanswered questions surround the most hotly contested hypothesis about how the pandemic began: the lab leak theory.

President Joe Biden confronted mounting pressure to probe the theory more seriously by announcing on Wednesday that he asked the intelligence community to complete an investigation into the pandemic’s origins within 90 days. His move came amid a dramatic shift in public opinion about a hypothesis once dismissed by the media as a distraction — only hours after news reports that his State Department had shut down a Trump-era investigation into whether COVID-19 accidentally escaped from a Chinese laboratory.