Very real evidence, collected by people who knew what they were doing, and relying on reliable knowledge and reasoning techniques to connect the dots. I, for example, would not have been able to go stare at the hole in the ground and say "yup, I know what did that." It required people developing the evidence, using the appropriate techniques to do so, and presenting their methods and conclusions. I'm not saying the conclusions aren't convincing. Far from it. I'm just saying that's how pretty much any serious work is done. I'm not sure how many situations I could even think of where that sentence wouldn't carry a whiff of hubris.
The first criminal charges have now been filed on this 737MAX issue, against an executive who was also the chief technical pilot for this program. He is indicted for fraud, accused of deceiving the FAA during the certification process: Former Boeing executive indicted for fraud in 737 Max tragedy - CNN This pilot's name figured prominently in last month's PBS Frontline episode: Boeing’s Fatal Flaw | Watch S2021 E16 | FRONTLINE | PBS | Official Site