The theory started from this report from August 2019 by the New York Times:

https://www.nytimes.com/2019/08/05/health/germs-fort-detrick-biohazard.html

Just realized that the above is behind paywall, here is a version that is not behind paywall
https://web.archive.org/web/20190806000002/https://www.nytimes.com/2019/08/05/health/germs-fort-detrick-biohazard.html