In the article, the authors say that, in 2015, US researchers created a chimeric virus from a bat coronavirus to infect mice lung cells. They tested the virus on human lung cells and predicted that it would be very dangerous to humans. They stopped doing research on it because it would break the US rules against making human viruses more powerful. They were working with virus researchers from Wuhan. The authors conjecture that the Wuhan researchers used the same techniques and made Sars-cov-2 and it accidentally got out.