They found unpublished dissertations from 2019 and 2020 that allegedly discussed the effects of coronaviruses on the human body. 

Additionally, uncovered materials revealed Chinese scientists had 'an unusually large amount of knowledge about the supposedly novel virus available at an unusually early stage.'

Based on the materials BND agents found and analyzed, they used a 'Probability Index' to measure the reliability of information, which determined the lab-leak theory was 'probable' with an '80 to 95 percent' certainty.